POLYZOA. 



557 



rudimentary polypide bud (Pliyladolaemata, see p. 551). It is not 

 certain where fertilization is effected, but it is generally described 

 as taking place in the body-cavity, and the eggs or larvae escape 

 either by the intertentacular canal, or by the rupture of the body- 

 wall of the parent, or by the degeneration of its polypide. In 

 Alcyonidium duplex the sexual zooecia possess two polypides ; the 

 first of these produces spermatozoa and then undergoes disruption 

 into a brown body, whereupon a second polypide is formed which 

 produces ova. 



In the oviparous forms, to whatever sub-order they belong, in 

 which the early 



, fi 



takes 



va 



coa 



development 

 place in the sea, a 

 free-swimming larva 

 with a functional 

 alimentary canal is 

 rapidly produced. 

 This larva has, gener- 

 ally speaking, always 

 the same form, and 

 is called Cyplwnautes. 

 On the other hand, 

 in all forms in which 

 the early stages are 

 passed through under 

 the protection of the 

 parent, the larva is 

 without a functional 

 alimentary canal, and 

 the other larval organs 

 present varying de- 

 grees of development 

 in different cases. 



Cyphonautes (Fig. 446) has the form of a laterally compressed 

 bell with a circle of cilia round the base. The base of the bell is 

 the ventral surface ; it has two openings both enclosed by a special 

 lobe of the ciliated ring. The larger and posterior of these leads 

 into a depression called the vestibule ; the smaller and anterior into 

 an ectodermal depression called the pyriform organ (pi}. The vesti- 

 bule is divided into two parts, the anterior chamber (va), at the 

 bottom of which is the mouth, and a posterior chamber (vp), into 



of, 



FIG. 440. Optical section of a Cyphonaiites (larva of Mem- 

 branipora or of Alcyonidium albidum). (From Perrier, after 

 Prouho.) ad adhesive organ ; coa anterior, cop posterior part 

 of the ciliary ring (velum) ; coq chitinons valves of the shell ; 

 ex ectoderm ; cs stomach ; fa anterior pit ; fc cavity of pyri- 

 form organ ; ma adductor muscle of the shell-valves ; mn 

 musculo-nervous tract ; ob aboral disc ; or entry into oeso- 

 phagus ; ph pharynx; pi pyrifonn organ ; pi vibratile tuft 

 of pyriform organ ; q horns of the adhesive organ; r rec- 

 tum ; va anterior, vp posterior chamber of vestibule. 



