528 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



void of a common cloaca ; and the zoOids which compose it 

 may arrange themselves into one or several circles, each of 

 which then acquires its cloaca. 



It thus appears that, in Botryllus, the ascidiozooid which 

 results from the metamorphosis of the caudate larva serves 

 merely as a kind of stock, from whence the other zoQids 

 which build up the ascidiarium proceed ; and this leads to 

 the still more singular process of development in Pyrosoma, 

 in which the first-formed embryo attains only an imperfect 

 development, and disappears after having given rise to four 

 ascidiozooids. 



In Pyrosoma^ the ovisac is attached by a short oviduct to 

 the walls of the atrium, into which it eventually opens, and 

 thus allows of the entrance of the spermatozoa. 



Of the process of yelk-division I could see nothing in my 

 specimen, which was preserved in spirit, but it has since been 

 traced, in fresh specimens, by Kowalewsky,^ who compares it 

 to that which takes place in osseous fishes. The result is the 

 formation of an elongated, flattened blastoderm, which occu- 

 pies one pole of the Q^^^ and is converted into what I termed 

 the cyathozooid^ which is shown by Kowalewsky to be a sort 

 of rudimentary ascidian (Fig. 150, VIII.). From this a pro- 

 longation or stolon is given off, which becomes divided by lat- 

 eral constrictions into four portions, each of which gives rise 

 to a complete ascidiozooid. As these increase in size, they coil 

 themselves round the cyathozooid, with tlieir oral openings 

 outward and their cloacal openings inwarrl, and thus lay the 

 foundation of a new ascidiarium (Fig. 150, VIII.). The cyatho- 

 zooid eventually disappears, and its place is occupied by the 

 central cloacal cavity (Fig. 150, IX.). Thus, in Pyrosoma, the 

 usual first stage of an Ascidian — the caudate larva — is abor- 

 tive, and serves only to found the colony by the buds which 

 are developed from it. 



In the pelagic genus Poliolum^ the cycle of life of the 

 species is represented by distinct sexual and sexless forms. 

 The egg produced by the sexual form (A)^ gives rise to a 

 caudate larva which passes into the first sexless form (B) ; 

 this gives off from the neural side of the body an outgrowth 



' " Ueber die Entwickelungsgescliiclite der Pvrosoma." (" Archiv fur Mikr. 

 Anatomie," 1875.) 



2 Iluxlev, " Remarks upon AppenrUcvlnria and Doliolvmy (" Phil. 

 Trans.," 1851.) Krohn, " Ueber die Gattmiff Doliohim,:'' (" Archiv fiir Natur- 

 ^eschichte," 185'2.) Gecrenbaur, " Ueber die Entwickelung von DoUolum.^* 

 {Zeitmhrift f>ir u'ift.9. Znologip^ 1853.) 



8 Keferstein and Ehlers, " Zoologische Beitriige," 1861. 



