The Pluteus of Laganum sp. 141 



unlike that described for other related forms. Goto (2), in discussing 

 Asterias pallida, has maintained that "the plane of bilateral symmetry 

 of the adult coincides with that of the larva, and that the oral side 

 of the former is the anterior side of the latter and the aboral side the 

 posterior." This is almost the case in Laganum, where the anlage of 

 the mouth of the adult is located in the anterior part of the ventral 

 surface of the larva. In Asterias pallida this condition comes about by 

 a gradual growth of the organs of the left side towards the median line. 

 If the amniotic cavity of Laganum is formed in this manner it must be 

 accomplished at an exceedingly early period, since in the specimens 

 under observation no trace of any such previous condition remains. 

 Moreover, if the formation of the amniotic cavity has been accom- 

 plished by a separate invagination from the left side, the subsequent 

 opening of the mouth into it presents an additional vexatious question. 



THE ENTEROCCELES. 



The median position of the hydroccele is also in striking contrast with 

 the location of the corresponding structure in other echinoderms, in all 

 of which it is described as lying on the left side. Since in this stage of 

 Laganum all connection of the hydroccele with the anterior enteroccele 

 has been lost and no direct evidence remains as to its mode of develop- 

 ment, it seems unwarranted to ascribe to it an origin differing from that 

 of other known forms. The structures that I have called the posterior 

 enteroccele are still more unusual, not only on account of their different 

 position, but also because of their striking dissimilarity in form from 

 those of other plutei. They lie as two entirely separate spherical 

 vesicles, ventral to the stomach on either side of the posterior lobe of the 

 hydroccele. That they have had a common origin seems highly prob- 

 able from their close resemblance to each other as well as from the fact 

 that they stand in similar relation to the spines on the oral surface of 

 the embryo. In many instances these spines, passing directly through 

 the wall intervening between the embryo and the amniotic cavity, lie 

 with their tissues in direct contact with the lumen of the vesicles. The 

 same condition occurs in forms where one of the vesicles has divided, 

 and from the smaller anterior one other spines seem to emerge. 



That no structures are present corresponding to the right hydroccele 

 and right posterior enterccele is rather remarkable considering the 

 advanced stage of development of those of the opposite side. The 

 undivided condition of the anterior enteroccele is less unusual, though 

 not of common occurrence in other closely related forms. According to 

 Bury (1), Echinoids and Ophiuroids possess two separate enterocceles 

 which lie at the sides of the oesophagus. Moreover, among the Asteroids 

 the same condition obtains in the Bipinnaria. However, in the older 

 stages of these forms a secondary fusing often occurs, especially in the 

 region of the preoral lobe, so that a common cavity comes to be present. 



