lO 



COMPARATIVE ANATOMY 



pared with the trochophore larva of annelids, but their ontogenetic 

 development does not support this assumption. 

 Eight hundred and fifty species are recognized. 



psajoococuM 



DIAGRAM OF A ROTIFER. 



Pig. 7. — Diagram of a Rotifer. The Rotifers are aberrant Proterostomians — and 

 one of the few invertebrate phyla which have not been suspected of being ancestors of 

 vertebrates. (Redrawn after Stempell.) 



Phylum 8. ECHINODERMATA 



The echinoderms include the star-fishes, sea-urchins, sea-cucumbers, 

 and sea-lilies. They are marine coelomate invertebrates with spiny 

 skins and a water- vascular system which they fill with sea-water. Super- 

 ficially radial in symmetry, the echinoderms are actually bilaterally 

 symmetrical animals which undergo metamorphosis from a bilaterally 

 symmetrical larva. Since the blastopore becomes the anal opening and 

 the mouth is formed secondarily, they are placed among the deuter- 

 ostomia, the group to which the chordates belong. The coelom is a true 

 enterocoel, that is, develops as an outpocketing of the enteron. 



The nervous system consists of a circumesophageal nerve ring from 

 which five nerves radiate. In addition, there is an aboral nervous system. 

 While most echinoderms possess a lacunar blood system, some have radial 

 blood-vessels. Specialized excretory organs seem to be wanting. The 

 sexes are separate and the gonads are interradial in position, with external 

 apertures on the aboral side of the animal. Respiratory organs are 

 varied, among them being dermal papillae which contain outpocketings of 

 the coelom. 



Some taxonomists recognize 10,000 species. 



Phylum 9. Annelida 



The annelids include earthworms, leeches, clam worms, and many 

 marine forms. All are metameric, coelomate, and without jointed legs. 

 True nephridia, segmental " and ectodermal, are present. In many, the 

 body-wall is beset with bristles or setae. The nervous system is meta- 

 meric and consists of supraesophageal and subesophageal ganglia, with 

 circumesophageal connectives and a ventral series of ganglia. 



Circulation is through a closed system of blood vessels. Blood is 

 pumped forward through a contractile dorsal aorta, and thence around 



