68 



IXVEKTEBRATE AXIMALS. 



than the ordinary Lydrotliecj?. As a general rule, each capsule in 

 the Sertidarida contains a number of the reproductive liuds, -which 

 are all borne upon a central axis which luns \ip the middle of the 

 capsule, anil is derived from the cnenosarc. 



In one di\isi(jn of this group — often described as a separate order, 

 under the name of Vaiiijianuhirida — some points of difference are 

 observable. In the typical Sertularians the little cups or hydrothecse 

 for the polypites are placed on the sides of the liranches, and they 

 are not stalked (fig. 34, a'), whilst the rejjroductive elements are 

 pioduced in fixed buds. In the Camprimdarida, on the other hand 

 (fig. 34, ?)), the hydrotheca^ are supported upon stalks, and are placed 

 at the ends of the branches, whilst the generative buds are usually 

 detached to lead an independent existence. In these forms the 

 reproductive zooids or gonophores start as simple buds ; but they 

 become gradually developed into free-swimming meilusoids, such 

 as have been 1 lefore alluded to. Each medusoid consists of a little 

 transparent glassy bell, from the under 

 surfjice of which there is suspended a 

 modified polj'pite, in the form of a 

 mamdjrium (fig. 36, a). The whole or- 

 ganism swims gaily through the water, 

 priijjelled by the contractions of the 

 bell or disc ; and no one would suspect 

 now that it was in any ^^'ay related to 

 the fixed jilant-Iike zoophyte from which 

 it was originally builded off. The cen- 

 tra,l polypite is furnished with a mouth 

 at its distal end, and the mouth opens 

 into a digestive sac. From the proximal 

 end of this stomach proceed four radiat- 

 ing canals which extend to the margins 

 of the bell, where they all open into a 

 circular vessel which runs round the 

 mouth of the tiell. Fi'om the circumfer- 

 ence of the liell hang also a number of 

 delicate extensile filaments or tentacles; 

 and the margin is further adorned with 

 a series of briglitly coloured spots, 

 wliich are ]iroba,b]y i-ndimentary organs 

 of vision and hearing. The mouth of 

 the l)ell is jiartially closed by a delicvite traiisj)arent membrane or 

 slii-jf, the si)-ca,lled "veil." Thus constitut(>d, the.se beautiful little 

 beings lead an i)ulependent a,nd locomotive existence for a longer 

 or shoiii'i- jierind. Ultimately, ova ami sperm-cells are ]iroduced in 



Fig. 31.— Gonnphnro nt , 

 tin; Cu/iilpiiimluriiUt. 



