146 SECTION A. GENERAL ZOOLOGY. 



Bury^ has given reasons for supposing that the internal madreporite 

 of the adult is the representative of the axial sinus, but neither in 

 adult nor larva is there apparently any trace of a right hydrocoele — 

 another added to many reasons for believing Holothurids to be the 

 most modified group of Echinoderms. 



If the main factor in Echinoderm metamorphosis be the pre- 

 ponderant growth of one side, we have still to suggest how a 

 free-swimming animal came to lose its symmetry. 



Whilst discussing the fundamental agreement in type of the 

 Echinoderm larvae, the Crinoids were left on one side. This was 

 necessary, because the only Crinoid the development of which 

 is known has a very modified life history. The free-swimming 

 existence of the larva of Antedon is very short and its ciliated 

 bands are not comparable to those of other Echinoderm larvae : 

 these bands may however have been derived phylogenetically from 

 the breaking up of a longitudinal ring, on similar lines to those on 

 which the Holothurid pupa has developed transverse bands out of 

 a longitudinal ring. In the Crinoid larva the alimentary canal is 

 a blind sac, but a larval stomodaeum is present as a blind pouch by 

 means of which we can determine the position of the mouth. This 

 is found to be situated in a corresponding position to that of the 

 Asterid larva ; in both cases there is a long praeoral lobe, which 

 contains a distinct anterior body cavity. 



As is well known the Crinoid larva fixes itself by the anterior 

 extremity of the praeoral lobe, this position of the body becoming 

 converted into the stalk, whilst the stomodaeum is slowly displaced 

 backwards until it reaches the posterior end of the larva, and 

 a young stalked Crinoid is developed. In Asterina gibbosa I have 

 shown that the larva also fixes itself by the apex of the praeoral 

 lobe which is likewise converted into a stalk. A new mouth 

 is formed to the left of the old one and, as metamorphosis proceeds, 

 the remains of the stalk are found to be inserted close to the mouth 

 within the hydrocoele ring, whereas in the Crinoid the stalk is 

 inserted in the centre of the aboral pole. Such a fixed stage with a 

 similar stalk probably occurs in the development of most Asterids: 

 it has been recognised in a typical Bipinnaria by Bury and in 

 every case where a shortened and modified development has 

 brought the later stages under observation. I need only mention 

 the Antarctic Asterias spirabilis described by Perrier, Henricia 

 (Echinaster) sanguinolenta discovered by Sars and Miiller, Asterias 

 Mijlleri and several others. Seitaro Goto it is true denies that 

 Asterias pallida has a fixed stage, but as the larva possesses an 

 apparatus similar to that used by other species in fixation it 

 is possible that the conditions of artificial culture prevented fixation 

 being accomplished. The general occurrence of a stalk in Asterids 

 would explain the presence of an immense praeoral lobe in the 



^ "The Metamorphosis of Echinoderms," by H. Bury. Quart. Joum. Micr. Sc. 1895. 



