502 



POLYZOA AND COMPOUND TUNICATA. 



it), waiting for expulsion. In this position they receive the 

 fertilizing influence from the testis, q, which discharges its pro- 

 ducts by the long spermatic canal, r, that opens into the cloaca 

 at /. At the very bottom of the post-abdomen, we find the 

 heart, o, enclosed in its pericardium, o'. In the tribe we are 

 now considering, a number of such animals are imbedded toge- 

 ther in a sort of gelatinous mass, and covered with an integu- 

 ment common to them all ; the composition of this gelatinous 

 substance is remarkable as including " cellulose," which gene- 

 rally ranks as a purely vegetable product. The mode in which 

 new individuals are developed in this mass, is by the extension 

 of " stolons" or creeping stems from the bases of those previ- 

 ously existing ; and from each of these stolons several buds may 

 be put forth, every one of which may evolve itself into the like- 

 ness of the stock from Avhich it pi'oceeded, and may in its tui-n 

 increase and multiply after the same fashion. A communication 

 between the circulating systems of the different individuals is 

 kept up, through their connecting stems, during the whole of 

 life ; and thus their relationship to each other is somewhat like 

 that of the several polypes on the polypidom of a Onmpanularia 

 (§304). 



332. In the family of Didemnians, the post-abdomen is absent, 



the heart and generative 

 F'"- ~'*^- apparatus being placed 



by the side of the intes- 

 tine in the abdominal 

 portion of the body. 

 The zooids are fre- 

 quently arranged in 

 star-shaped clusters, 

 their anal orifices being 

 all directed towards a 

 common vent which oc- 

 cupies the centre. This 

 shortening is still more 

 remarkable, however, in 

 the family of Botri/Uians, 

 whose beautiful stellate 

 gelatinous incrustations 

 are extremely common upon sea-Aveeds and submerged rocks 

 (Fig. 249). The anatomy of these animals is very similar to 

 that of the ylm(?;w(w/TO already described; with this exception, 

 that the body exhibits no distinction of cavities, all tlie organs 

 being brought together in one, which must be considered as 

 thoracic. In this respect, there is an evident approximation 

 towards the solitary species. 



333. This approximation is still closer, however in the " social" 

 Ascidians, or Qlavellinidce ; in which the general plan of structure 

 is nearly the same, but the zooids are simply connected by their 



iteeiK^-' 





Botryllus violacit.t : — a, clnsieron the purfuce of a Faeus 

 B, portion of the siiiiii.' enlarged. 



