LARVAL FORMS. 



307 



Fig. 223. A. The lakva of a Holo- 



THUROID. 



B. The larva of an Asteroid. 



m. mouth; sr. stomach; a. anus; I.e. 

 primitive longitudinal ciliated band; pr.e. 

 prseoral ciliated band. 



The alimentary tract is formed of the three typical divisions. 



The body cavity is not developed directly as an outgrowth of the 

 alimentary tract, though the pro- 

 cess by which it originates is very 

 probably secondarily modified 

 from a pair of alimentary out- 

 growths. 



Paired excretory organs, open- 

 ing to the exterior and into the 

 body cavity, are often present 

 (fig. 226 nph). 



This type of larva is found in 

 the Rotifera (fig. 217) (in which 

 it is preserved in the adult state), 

 the Chaetopoda (figs. 225 and 

 226), the Mollusca (fig. 218), the 

 Gephyrea nuda (fig. 227), and 

 the Polyzoa (fig. 228) \ 



4. Tomaria. — This larva 

 (fig. 229) is intermediate in most 

 of its characters between the 



larvae of the Echinodermata (more especially the Bipinnaria) and the 

 Trochosphere. It resembles Echinoderm larvae in the possession of 

 a longitudinal ciliated band (divided into a praeoral and a postoral 

 ring), and in the derivation of the body cavity and water-vascular 

 vesicle from alimentary diverticula ; and it resembles the Trocho- 

 sphere in the presence of sense organs on the praeoral lobe, in the 

 existence of a perianal ring of cilia, and in the possession of a con- 

 tractile band passing from the praeoral lobe to the oesophagus. 



5. Actinotrocha.— The remarkable larva of Phoronis (fig. 230), 

 known as Actinotrocha, is charac- 

 terised by the presence of (1) a post- 

 oral and somewhat longitudinal cili- 

 ated ring produced into tentacles, and 

 (2) a perianal ring. It is provided 

 with a praeoral lobe, and a terminal 

 or somewhat dorsal anus. 



6. The larva of the Brachio- 

 poda articulata (fig. 220). 



The relationships of the six types 

 of larval forms thus briefly charac- 

 terised have been the subject of aeon- Fig. 224. A larva of Steongy- 

 siderable amount of controversy, and i-ocentkus. (From Agassiz.) 

 the following suggestions on their af- »«• mouth; a. anus; o. oesophagus; 



finities must be viewed as somewhat ti.^t^'f^i, """" w^ j:™."!" 



speculative. The Pilidium type of tube ; r. calcareous rods. 



^ For a discussion as to the structure of the Polyzoon larva, vide Vol. i. p. 253. 



20—2 



