398 



Prof. E. W. MacBride. 



[Nov. 7, 



long, and the " antero -lateral " arms (or " oral " arms, as German writers 

 term them), which are the next to develop, are about as long. It may be 

 incidentally remarked that all writers who, up till now, have dealt with the 

 hybridisation of Echinoderms (with the exception of Shearer, De Morgan, 

 and Fuchs) have ignored all the stages in the development later than the 

 four-armed larva. To judge from mucli of what has been written on this 

 subject, no one would ever suspect that the larva of an Echinoid had more 

 than four arms ; but, as represented in our figure, the six-days-old larva of 

 Echinocardium cordatum possesses in addition a pair of " postero-dorsal " 

 arms, and in front of the mouth a pair of small elevations are to be seen 

 which are the first rudiments of the " prae-oral " arms. The oesophagus and 

 the stomach and the ccelomic sacs lying at the sides of the oesophagus are 

 clearly visible. From the aboral pole of the larva a club-shaped 

 appendage projects backwards. This is the distinctive feature of the 

 larva of Echinocardium and its allies. So far as our present knowledge goes 

 it appears to be characteristic of the larvre of Spatangoidea generally. 

 Turning now to the consideration of the larval skeleton, we note that 

 each post-oral arm is supported by a lattice-work consisting of parallel 

 calcareous rods bound together by numerous cross-bars, but that each antero- 

 lateral arm is supported by a single calcareous rod. The skeletons of both 

 the antero-lateral and the post-oral arm on each side are formed as out- 

 growths of the " primary calcareous star " which appears on each side of the 

 gastrula in all Echinoida. This star sends back a third process towards the 

 aboral pole of the larva, which is known as the " body- rod." This is situated 

 beneath the stomach. Dorsal to the stomach from the skeleton of the antero- 

 lateral arm there is given off a rod which runs towards the aboral pole 

 parallel to the body-rod, but which does not reach (as yet) so far. This is 

 termed the " recurrent rod." The skeleton of each postero-dorsal arm is also 

 a lattice-work of parallel rods connected by cross-bars, but it originates 

 from a lateral centre of calcification entirely distinct from the primary star. 

 The skeleton of the " aboral spike " owes its origin to the secretory activity of 

 a group of mesenchyme cells wedged in between the ends of the body-rods, 

 which is clearly visible in the larva when it is two days old before the 

 antero-lateral arms have developed, or any external trace of the aboral spike 

 has appeared. The skeleton consists of a lattice work of three slightly 

 diverging calcareous rods bound together by cross-pieces aud beset externally 

 by spines. This lattice work is connected with the ends of the body-rods. 

 The aboral spike bears at its apex a cap of columnar epithelium carrying long 

 cilia. The epithelium covering the rest of it is thin, flat, and non-ciliated. 

 We may compare with this larva the larva of Echinus escidentus of the 



