ECHINODERMATA. 561 



tentacles which form the apex of the radial water-vascular 

 tubes. 



It has already been mentioned that according to Metschnikoff (No. 560) 

 a new oesophagus is formed which perforates the water-vascular ring, and 

 connects the original stomach with the original mouth. Agassiz (No. 543) 

 maintains that the water-vascular ring grows round the primitive oesophagus. 

 He says " During the shrinking of the larva the long oesophagus becomes 

 " shortened and contracted, bringing the opening of the mouth of the larva 

 " to the level of the opening of the oesophagus, which eventually becomes 

 "the true mouth of the starfish." The primitive anus is believed by 

 Metschnikoff to disappear, but by Agassiz to remain. This discrepancy 

 very possibly depends upon these investigators having worked at different 

 species. 



There is no doubt that the whole of the larval organs, with 

 the possible exception of the oesophagus, and anus (where absent 

 in the adult), pass directly into the corresponding organs of the 

 starfish and that the prae-oral part of the body and arms of the 

 larva are absorbed and not cast off. 



In addition to the Bipinnarian type of Asteroid larva a series of other 

 forms has been described by Miiller (No. 561), Sars, Keren, and Danielssen 

 (No. 554) and other investigators, which are however very imperfectly 

 known. The best-known form is one first of all discovered by Sars in 

 Echinaster Sarsii, and the more or less similar larvae subsequently investi- 

 gated by Agassiz, Busch, Miiller, Wyville Thomson, etc. of another species 

 of Echinaster and of Asteracanthion. These larvae on leaving the egg have 

 an oval form, and are uniformly covered by cilia. Four processes (or in 

 Agassiz' type one process) grow out from the body ; by these the larvae fix 

 themselves. In the case of Echinaster the larvae are fixed in the ventral 

 concavity of the disc of the mother, between the five arms, where a tempo- 

 rary brood-pouch is established. The main part of the body is converted 

 directly into the disc of the young starfish, while the four processes come to 

 spring from the ventral surface, and are attached to the water- vascular ring. 

 Eventually they atrophy completely. Of the internal structure but little is 

 known ; till the permanent mouth is formed, after the development of the 

 young starfish is pretty well advanced, the stomach has no communication 

 with the exterior. 



A second abnormal type of development is presented by the embryo of 

 Pteraster miliaris, as described by Koren and Danielssen 1 . The larvae to 

 the number of eight to twenty develop in a peculiar pouch on the dorsal 

 surface of the body. The early stages are not known, but in the later ones 

 the whole body assumes a pentagonal appearance with a mouth at one edge 



1 The following statements are taken from the abstract in Bronn's Thierreichs. 

 B. II. 36 



