622 INVERTEBKATA CHAP, xvn 



a cup on a stalk, carrying in its apex a spherical pigmented concre- 

 tion, the otoli th. The hinder wall of the brain-vesicle is thick, and forms 

 a ganglion, termed the visceral ganglion (gang.visc, Fig. 454, F). 

 There is a protuberant region which we may term the chin (inn, Fig. 

 451). Below the mouth the ectoderm develops three papillae of 

 glandular cells, one median and two lateral, which are the organs of 

 fixation. 



Although the larva has a mouth it cannot feed, because this is 

 closed by a gelatinous cuticle termed the test or mantle. The test 

 is secreted by the ectoderm and enswathes the whole body, covering 

 over both mouth and atrial openings. It contains certain cells called 

 test cells, which are budded from the ectoderm (t.c, Fig. 451), but 

 these cells are to be carefully distinguished from the cells bearing 

 the same name which surround the egg ; these latter, as we have 

 seen, are rudimentary ova. f 



After a short free life the larva fixes itself to the substratum by 

 the papillae on its chin, and then undergoes a rapid degenerative 

 metamorphosis. By the contraction of the ectoderm of the tail, the 

 notochord, nerve cord, and muscles are bent and broken into 

 fragments, which are rapidly devoured by wandering cells (Fig. 

 453). The ectoderm becomes invaginated so as to form pockets, and 

 these pieces of intucked ectoderm are attacked and devoured by 

 wandering cells ; and by these processes the whole of the tail is 

 disposed of. The chin region grows enormously, and by this growth 

 the mouth is carried upwards, away from the substratum to which 

 the animal is attached. This process is similar to the one by which 

 the mouth is rotated upwards in the metamorphosing larvae of Cirri- 

 pedia, Polyzoa, Entoprocta, and Crinoidea ; and it occurs for the same 

 reason in all four cases, viz. from the physiological necessity of lifting 

 the mouth up so as- to put it into a better position for catching 

 floating plankton. 



Willey (1893) has followed in some detail the metamorphosis of 

 dona intestinalis, and his results have been confirmed by other 

 workers. He finds, as already mentioned, that when the metamor- 

 phosis begins the neuropore is reopened; and he suggests, what is 

 a priori extremely likely, that this opening has persisted, in a potential 

 form, as union of the ectodermic wall of the brain-vesicle with the 

 ectoderm all through the larval life. During larval life a gutter, 

 which we may term the hypophysial canal (hyp, Fig. 454), is 

 formed in the dorsal wall of the brain- vesicle. This gutter becomes 

 completely grooved off from the vesicle behind, where it is continuous 

 with the cavity of the spinal cord, but opens into the vesicle in front. 



At the beginning of metamorphosis, when the neuropore is 

 reopened, the hypophysial canal becomes continuous with this pore, 

 and the rest of the brain-vesicle becomes completely grooved off from 

 the canal. The brain-vesicle then rapidly degenerates (Fig. 455), 

 and the sense-organs are broken up and absorbed. The adult 

 ganglion is formed as a new proliferation (Figs. 454, 455) from the 



