V. Echinoidea. \ 9 



V. Echinoidea. 



See also Barrois, supra, 2, Neumayr, supra, p 5, the Sarasins ( 3 ), supra, 

 p 4, and Semon, supra, p 3. 



Fleischmann describes the cleavage of the egg in Echinocardium cordatum. After 

 impregnation the ovum becomes elliptical, and grooves appear on its flattened 

 sides, one of which, at the animal pole, is more marked than the other and 

 reaches the centre sooner; while the 2 resulting blastomeres are unequal. The 

 second cleavage plane appears an hour later, perpendicular to the first one, and 

 divides the smaller blastomere before the large one, as in Asterina gibbosa. A flask- 

 shaped blastocoel is formed between the 4 blastomeres, open at both ends, 

 but largest at the vegetative pole. It increases in size and the equatorial plane 

 appears, cutting first the 2 smaller and then the 2 larger quadrants. The next 2 

 planes are parallel to it (isoplanes), the first formed, near the animal pole, 

 cutting off 4 small polar cells , while 4 large vegetative cells are subsequently 

 formed at the lower pole. The 6 th and 7 th planes are vertical (orthoplanes), 

 but only cut the 2 middle tiers of cells, making 24 in all. Two more isoplanes 

 cut the polar cells and bring on the stage of 32 cells, consisting of two 8-celled equa- 

 torial rings and four 4 -celled polar groups. After the blastula leaves the vitel- 

 line membrane, mesenchym-cells are budded off into the funnel-shaped 

 blastocoel from 4 cells at the lower pole which are the remains of the 4 first 

 vegetative blastomeres , after cutting by successive isoplanes. The mesenchym- 

 cells assume a bilateral symmetry , but finally spread over the interior of the 

 blastula which passes gradually into the gastrula. - - E. thus shows the extreme 

 stage of a cleavage process which occurs in other Echinids , but has no phyloge- 

 netic significance. 



Prouho denies the existence of a circum-anal ring uniting the ducts of the 

 genital glands in Echinus, as described by Perrier. Both in this genus and in 

 Dorocidaris the genital ducts are mutually independent; but they are held to- 

 gether ,by a pentagonal extension of the mesentery which unites the gullet and 

 ovoid gland, and it contains a network of lacunae which are connected with the 

 visceral blood-lacunae. The ambulacral nerve of D. papillata consists of 2 

 parts, an outer one of connective tissue (often pigmented) and an inner one of 

 nerve fibres and cells. The proximal end of the latter passes into the pharyngeal 

 epithelium, while the former is continuous with that of the peristomial lip. An 

 intra-neural space, perhaps due to reagents, may appear between them. 

 Both parts share in the formation of the tentacular nerves , the outer one acqui- 

 ring nervous elements; and having passed through the more proximal of the 2 

 tentacular pores, the outer one spreads out to form the external nervous plexus, 

 while the inner one enclosing an intra-neural space supplies the tentacle. The 

 radial nerve is traversed vertically by supporting fibres of connective tissue, and 

 a few nerve cells are scattered among its fibres, the majority, however, being on 

 the outer face of its inner portion. Towards the distal end of the ambulacrum of 

 Echinus acutus the nerve band gradually comes to enclose the water- vessel com- 

 pletely, and its internal portion joins with those of adjacent ambulacra to form 

 an aboral nervous ring. This lies along the outer edge of the membrane uniting 

 the genital ducts which it innervates. This structure also occurs in Strongylo- 

 centrotus lividus, but not in D. papillata. - - In addition to the 3 kinds of amoe- 

 boid cells (colourless, refractile, and brown) described by Geddes [see Bericht 

 for 1880 I p 265] the coelomic fluid contains spherical cells each provided with 

 a long cilium. The amoeboid cells occur in all the tissues including the organic 



