NEMERTEA AND ROTI FERA 237 



The nemerteans are classified as follows : 



Palaeonemertini. Proboscis without stylets; cerebral ganglia and 

 lateral nerves in the ectoderm or between the two layers of muscles, 

 Carinella. 



Metanemertini. Proboscis armed with stylets; lateral nerves 

 within all the muscle layers. Tetrastemma, Geonemertes^ Malacobdella. 



Heteronemertini. Proboscis without stylets; a second layer of 

 longitudinal muscles outside the circular muscles ; lateral nerve cords 

 lie between the two. Linens, Cerebratulns. 



PHYLUM ROTIFERA 



Minute animals, unsegmented and non-coelomate, typically with a 

 ciliated trochal disc for locomotion and food collection, a complete ali- 

 mentary canal with anterior mouth and posterior anus, and a muscular 

 pharynx with jaws of a special type ; excretory system with flame cells 

 joining the hind gut to form a cloaca ; no blood system or respiratory 

 organ ; very simple nervous system ; sexes separate, two kinds of eggs, 

 one developing immediately without fertilization and the other, which 

 is fertilized, thick-shelled and developing only after a resting period. 



This group contains a large number of forms of great interest to 

 the microscopist which are easily obtained from many kinds of fresh 

 water. They are, generally speaking, the smallest of all metazoa. They 

 vary little in structure and present a remarkable similarity to the 

 trochosphere larva. It must be admitted that the Rotifera are on a 

 lower stage of organization than the annelids and molluscs which 

 possess this larva and may even be related to a common ancestor of 

 these phyla. On the other hand, the Rotifera come near to the Platy- 

 helminthes, the Gastrotricha and Nematodes. 



An elastic external cuticle covers most of the body. Under this is 

 a syncytial ectoderm ; a continuous layer of muscles forming a body 

 wall is absent (as in the Arthropoda), but isolated bands of muscle, 

 chiefly longitudinal, traverse the body (or perivisceral) cavity (Fig. 

 171). 



What is the true nature of the body cavity is a question which has 

 never been properly answered. It is a wide space between ectoderm 

 and endoderm, traversed by muscles, and is neither a coelom nor a 

 haemocoele in the narrower sense, but probably only a derivation of 

 the segmentation cavity of the gastrula (the blastocoele), as in the 

 trochosphere larva. But they do possess a body cavity and not a solid 

 parenchyma, and so differ from the Platyhelminthes. Their excretory 

 system is, however, very similar to that of the latter phylum, and in 

 the union of the excretory duct with the gut the rotifers resemble 

 certain specialized trematodes. 



