Campbell, Biology 6th Ed - Chapters 32 & 33, pgs 633-645, 656-671
Campbell, Biology 7th Ed - Chapters 32 & 33, pgs 626-636, 650-664
Symmetry – general, structured body plan. See radial and bilateral symmetry below.
Coelom – fluid-filled body cavity surrounded by mesoderm-derived tissue.
Triploblastic – having three layers of cells during development: the endoderm, ectoderm, and mesoderm.
Protostomes – coelemate animals whose embryogenesis includes the blastopore developing into the mouth of the animal.
Segmentation – division of the body into separate, sequential cavities.
Closed circulatory system – when blood is maintained in interconnected blood vessels and not mingled with other body fluids.
Cephalization – concentration of sensory and neural organs at the anterior end of an animal.
In the previous summary, Simple Animals, we began to explore Kingdom Animalia. In this summary we look at more animal phyla, those described as protostomes. Below is a review of the current classification of animals, based on morphology and development. You should familiarize yourself with the major anatomical and developmental differences that define animal phyla.
The traditional phylogeny is shown below. It is based on the following anatomical / developmental traits:
· Presence / absence of true tissues
· Type of body symmetry
· Presence / absence of body cavity (coelom)
· Pattern of coelom development (acoelom, pseudocoelom, true coelom)
The grades of animal evolution are as follows (italics – taxa described here):
1. No true tissues (only non-specialized cells) : Parazoa
2. True tissues (functionally-specialized cells) : Eumetazoa
A. Radial symmetry, diploblastic (2 germ layers during development) : Radiata
B. Bilateral symmetry, triploblastic (3 germ layers) : Bilateria
1) No body cavities or blood vascular system : Acoelomates
2) Body cavities and blood vascular system
a. Body cavity not enclosed in mesoderm : Pseudocoelomates
b. Body cavity enclosed in mesoderm : Coelomates
i) Early development includes spiral and determinate cleavage, mouth developing from blastopore, and schizocoely : Protostomes
Mollusca (snails, clams, octopi)
Arthropods (insects, spiders, shellfish)
ii) Early development includes radial and indeterminate cleavage, anus developing from blastopore, and eterocoely : Deuterostomes
(see Deuterostome lab summary)
This phylogeny of all animals is based on morphological differences and similarities.
Protostomes are bilaterally symmetrical (Bilateria) and triploblastic. These terms should be familiar to you from the previous lab.
Phyla within the Bilateria clade are classified according to the presence and development of a coelom (body cavity): acoelomates, pseudocoelomates, and coelomates. In the Simple Animals Lab, you learned about acoelomates and pseudocoelomates. Here, you will begin your survey of the coelomates.
Anatomy of coelomate body tissues: Body cavity is completely lined with mesoderm-derived tissue; it is a true coelom.
The two major types of coelomates are protostomes and deuterostomes. These groups differ in several characteristics of early development.
Location of Protosomes on the Morphological Phylogenetic Tree
Phylum Annelida (Clamworms and Earthworms)
Annelid worms are largely terrestrial and have segmented bodies, with the coelom partitioned via septa. Segmentation is an important evolutionary step towards specialization of body regions. The digestive tract, longitudinal blood vessels, and nerve cords run the length of the worm. However, each segment has its own set of excretory tubes, called metanephridia. Annelids have some cephalization, with a pair of cerebral ganglia at their anterior end. They also have a closed circulatory system, with muscular blood vessels to pump the blood through and around each segment. The well-developed coelom provides a hydrostatic skeleton. Below are examples of different types of Annelids.
Aquatic Annelid: Clamworm (Nereis). Note segmentation and parapodia with setae.
A common annelid is the earthworm.
Dissection of the head region of an earthworm
A cross-section reveals the coelomate nature of the earthworm.
Virtual Earthworm Dissections:
Phylum Mollusca (Clams, Chitons, Snails, Octopi)
Most mollusks are marine, although some live in freshwater or on land. They are soft-bodied, and most are protected by a shell. Unlike annelids, mollusks lack true segmentation. The majority of mollusks have four traits in common: 1) a hard external shell; 2) a mantle, which secretes the shell; 3) a visceral mass; and 4) a muscular foot used for locomotion. Many mollusks use a rasping organ, the radula to scrape food off the substrate. The familiar Classes of mollusks have distinctive body shapes.
Class Polyplacophora (Chitons)
Class Scaphopoda (the tusk shells)
This class is the largest molluscan class, containing the snails and slugs. Most gastropods have a single, spiraled shell, and distinct heads with eyes at the end of tentacles.
Turban Snail Dorid
The bivalves include the clams, oysters and mussels. They have shells divided into two halves, which connect at a dorsal hinge. Bivalves are aquatic and have gills for suspension feeding and for gas exchange. Water flows into the mantle cavity via the incurrent siphon and leaves via the excurrent siphon. Food trapped on the gills is conducted to the mouth using cilia. Most bivalves are sedentary. They have an open circulatory system and can move using their foot.
Cephalopods are markedly different from other mollusks. While still aquatic, cephalopods are mobile and can be very quick. The have much greater cephalization, with a complex brain, well-developed nervous system, large eyes, and the ability to learn complex tasks. Mobility and advanced nervous system are related to the cephalopod’s predatory nature. Cephalopods are also the only mollusks with a closed circulatory system. They have secondarily lost their shells (except in the nautilus). Their mouth is surrounded by tentacles and they bite their prey with a horny beak and inject a poison. The tentacles are used in locomotion and prey capture. A siphon is used to bring water into the mantle cavity for gas exchange via gills. The siphon is also used by squid for locomotion.
The squid Histioteuthis hoylei An octopus
A nautilus eating a crab
Phylum Arthropoda (Insects, Spiders, Crustaceans)
Arthropods are the most successful group of animals. They can be found in marine, freshwater, and terrestrial habitats. They have a hard, external skeleton (exoskeleton or cuticle) made from layers of protein and chitin. In order to grow, an arthropod must occasionally shed its exoskeleton, a process called molting. They also have jointed appendages and segmentation. Many body segments and associated appendages are specialized for a given function. Locomotion is achieved in many different ways, including wings, walking legs and swimming legs. Cephalization is extensive - there is a concentration of diverse sensory structures at the anterior end of the animal (eyes, antennae). All arthropods have open circulatory systems. Respiratory and excretory systems vary. Below are listed a few of the extant arthropod groups.
Class Merostomata (Horseshoe Crabs)
Arachnids include scorpions, spiders, ticks, and mites. All arachnids have eight legs (4 pair); they also have 2 additional pairs of appendages – the chelicerae (poison “fangs” for prey capture) and the pedipalps (sensory).
Scorpion under UV fluorescence Mite
Crustaceans are primarily aquatic and include crabs, lobster, shrimp, barnacles, crayfish, and many other animals. They have compound eyes and two pairs of sensory antennae. Locomotion is accomplished by walking with legs or swimming with their tails.
From your study of crayfish, you should know about the body structure of this animal – the segmentation, appendages, and functional specialization. Gas exchange is accomplished via gills, and the nervous system consists of an anterior brain and a ventral nerve cord. Also at the anterior end are the paired green glands, which are the excretory organs. Crayfish have a heart which pumps blood through the open circulatory system. Below are images of a dissected crayfish.
Class Chilopoda (centipedes)
Chinese Red Head centipedes
Class Diplopoda (millipedes)
Insects are the most diverse group of extant living organisms. Part of the success of insects is related to the evolution of flight. Many insects have one or two pair of wings attached to the dorsal side of the thorax segment. Insect wings are extensions of the cuticle rather than true appendages. Insects have complex organ systems. They have an open circulatory system, with a heart that pumps hemolymph around the hemocoel (a true coelom). Metabolic wastes are removed by the Malpighian tubules. A vast system of branching tubes, called the tracheal system, infiltrate the body and carry oxygen to the cells. The trachea open to the outside at the body surface via spiracles. The nervous system includes ventral nerve cords with several segmental ganglia. Below are images of the external and internal anatomy of the grasshopper.
-Compare the protostomes you studied with respect to the excretory and respiratory systems.
-What is the difference between an open and closed circulatory system? Which protostomes have a closed circulatory system? Which have an open circulatory system?
-Which protostomes are cephalized?
-Why is segmentation such an important evolutionary step?
-Assuming an open circulatory system in ancestral to protostomes, how many times did a closed circulatory system evolve in this group?
-What functions do appendages perform in cephalopods? In Arthropods?
-What are the different ways that arthropods move?
-What advantages does an exoskeleton provide? What challenges does it create?
-In the protostomes, compare all groups you studies according to locomotion, feeding habitat and nervous system. How do you think locomotion and feeding relate to the structure of the nervous system?