428 NOTES AND MEMORANDA. 



Echinodermata. 



Embryogeny of Asteriscus verruculatus. — Barrois has extended 

 his embryological investigations to the Asteroidra, of which group the 

 development of Asteriscus verruculatus is described,* He finds that 

 the gastrula is of the primitive type, that it grows into a harge, ciliated, 

 and completely closed sac, and then becomes trilobed ; the two side 

 lobes fall away and the young asterid is develojied from the median 

 ijortion. By invagination of the endoderm the enteric canal becomes 

 divided into three parts, the two lateral sacs being peritoneal, and 

 completely surrounding the middle one. The mouth, as in most, is 

 formed by the invagination of the ectoderm, which pushes its way in 

 between the two peritoneal sacs. The enteric cavity likewise gives 

 rise to the water-vascular system. 



The star-shaped form of body is due (1) to tlie flattening of the 

 rounded large (median) lobe of the embryo ; this is effected from 

 behind forwards, and aftects the intestine and the parts annexed thereto ; 

 (2), between the anterior and posterior surfaces thus formed there 

 arise ectodermal thickenings which grow into the arms ; of these, one 

 arises at the most anterior point, and the two others on either side ; 

 (3), the development of the water- vascular system into a ring round 

 the mouth. The peculiar point in the structure of this asterid at this 

 moment is the great degree of asymmetry which it exhibits, owing to 

 the much greater development of the superior as compared with the 

 inferior surface, and the exceutric position of the mouth ; this want of 

 symmetry gradually disappears, owing to the mode of growth now 

 followed ; as this goes on, the ambulacral lobes are seen to divide into 

 five branches ; at first three lobes are seen, and then five are formed 

 by the trifurcation of the median lobe. 



As time goes on, the intestine gives ojff five ca3ea, and becomes 

 provided with an anus ; and the calcareous plates take on a definite 

 arrangement. 



The most important points are henceforward associated with the 

 elongation of the arms and the increase in the number of the ambu- 

 lacral joints, which is effected in just the same way as is the multipli- 

 cation of the " zonites" of annelids. 



Comparing this form with those in which there are intermediate 

 larval stages, we may note that, in Asteriscus, the endoderm grows out 

 into a large sac, from which the intestine, peritoneal sac, and water- 

 vessels are directly differentiated, whereas in the others the endoderm 

 forms the intestine, and it is this intestine which gives rise to the 

 other organs ; in the young starfish three regions, dorsal, ventral, 

 and lateral can be made out ; the observation that the ambulacra 

 exhibit the same mode of increase as the Annelides may be applied to 

 the Echinoidea, as well as to the Aster oidea. 



On the Skeleton of the Asteriadse. — An article | on this subject, 

 by M. L. Viguier, consists of an historical introduction, a careful and 

 detailed description of the composing parts, and an account of a number 

 of genera. His chief conclusions are : — 



* ' Journ. Anat. et Phys.' (Kobin), xv. (1879) 1. 

 t 'Arch. Zool. Exper. et Gen.,' vii. (1878) 33. 



