530 IN VERTEBRATA CHAP. 



tion of the development of Synapta, and corrects many of Semen's 

 misstatements. It is the more to be regretted that Synapta has to 

 be chosen as type, because in its adult anatomy it is one of the most 

 aberrant members of the class Holothuroidea. 



The early development of Holothuria tubulosa has been described 

 by Selenka (1876); this species gives rise to a larva like that of 

 Synapta, but the adult is one of the least modified of Holothuroidea. 

 Selenka did not keep his larvae alive more than four or five days. 

 It is much to be desired that the development of this form should be 

 studied again. By using pure cultures of Nitschia as food the 

 larvae of Cucumaria saxicola have been reared throughout the whole 

 of their development at Plymouth. 



Selenka only found one ripe female Synapta amongst all the 

 hundreds which were brought to him by the collectors of the Naples 

 Station. It is probable that this was partly due to the season of year 

 at which he visited Naples, and partly to the fact that Holothuroids 

 are not so easily obtained in large numbers as are Asteroids, 

 Ophiuroids, and Echinoids, at any rate on British shores. Synapta 

 leads a burrowing life in sand and gravel, and is frequently not 

 reached by the dredge ; the most hopeful place to find it would be by 

 search during extreme low tides in sheltered inlets like the Clyde. 



The other genera, such as Holothuria, Cucumaria, etc., seem to hide 

 in crevices amongst stones, and only occasional stragglers are caught by 

 the dredge. Selenka (1876) secured large numbers of Holothuria by 

 a contrivance resembling a lobster trap, viz. a box with the lid fastened 

 down but with a small hole in the top, which he sunk in the sea. 



The segmentation of the egg of Synapta, as described by Selenka, 

 is the most regular as yet observed in the animal kingdom. The egg 

 divides into two and then four oval segments, and these divide into 

 two tiers of four each. At the next cleavage each cell divides by a 

 radial furrow into two daughters lying side by side ; in this way we 

 get two tiers of eight cells each. 



Already in the 4-cell stage the first trace of the blastocoele 

 made its appearance as a separation of the blastomeres from 

 one another in the centre of the egg; in the 32-cell stage, 

 which consists of four rings of eight cells each, these cells surround 

 a wide space open above and below (Fig. 394). In the following 

 cleavage each cell divides by a longitudinal furrow so that the number 

 of cells in each tier is doubled. The rings nearest the equator 

 continue to divide by alternate radial and meridional furrows, but in 

 the upper and lower tiers some of the cells migrate pole- wards and 

 form here rings of smaller diameter, so that the open spaces at the 

 poles are covered in and a closed blastula results. 



When 512 cells have been formed, that is at the conclusion of 

 the ninth cleavage, each develops a cilium and the blastula begins to 

 rotate within the egg-membrane. The blastula elongates, and a slight 

 thickening occurs at one pole since the cells in this region become 

 columnar. This thickening we may regard as a vestigial apical 



