LECTU'RES ON EMBRYOLOGY. 



21 



,yefc any ealetreous plates to support the isolated 

 spines, which rest only upon loose, calcareous nets 

 similar to those of the star-fish when first develop- 

 ed. With these changes in the main body, the ex- 

 ternal frame is gradually reduced, and finally en- 

 tirely lost. In this condition of the new animal, 

 when deprived of its transparent envelope (Plate 

 XL fig, G.) it is easy to recognise a young sea- ur- 

 chin, a young animal of which this figure, (Plate 

 XEII. fig B.) represents the animal in its per- 

 fect condition. 



[PLATE XIII SEA URCHINS J 



Muller has had an opportunity of tracing several 

 other larvse of the same kind, In one of them 

 there are appendages above as well as below, re- 

 sembling otherwise the first state of the Ophiura; 

 in another, similar in form to Plate XL, in which 

 there are, however, no crescent-shaped, vibrating 

 epaulettes ; in another still, there are two such cre- 

 scents only ; and in all of them there are hollow 

 spheres, with elongated tubes, and a seeming mouth 

 beneath; and all of them are transformed into 

 echini-like animals 



All these observations leave no doubt as to the 

 fact that certain embryonic echinoderms, observed 

 during summer, are protected by an external, com- 

 plicated frame work, which has not been noticed 

 in those observed during winter. In addition to 

 this, t mav mention that this external envelope re- 

 sembles very much the transparent body of some 

 jelly fishes, the Beroe for instance, which are also 

 provided with vibratory cilia, arranged in a pecu- 

 liar manner, and which move freely in the water. 

 And if we compare this curious condition of the 

 young Echini, with what is known of the growth 

 of medusas, where a simple egg will divide into 

 several masses to give rise to several individuals 

 we cannot be surprised that there are in the Echini 

 also, similar phenomena; and that a body, entirely* 

 different from the animal in its full grown condi- 

 tion, is developed, to nurse, as it were, the perfect 

 animal, and not to acquire in itself any peculiar, 

 prominentj final form, These phenomena of alter- 

 nate generation I shall illustrate more fully in the 

 next lecture. I merely allude to them now, in 

 order to suggest the probability of alternate sum- 

 mer and winter generations in echinoderms, dif- 

 fering from each other in the same manner as 

 ordinary alternate generations are known to 

 differ. 



The young Ophiura, the young Star-fish and the 

 young Echini, have not yet Been traced through 

 all their changes up to full grown animals. It was 

 the condition of their growth figured in these va- 

 rious diagrams, respecting which investigations 

 have been instituted; but the further investigation 

 was interrupted by the circumstances. But on 

 collecting along the shores small animals of those 

 species, and forming series of individuals from the 

 smallest size up to this perfect condition, the in- 

 vestigation of their growth and the changes which 

 they undergo, can be made out as completely as if 

 made upon one and the same animal during its 

 real growth. Indeed, most of the changes noticed 

 in the above described larvee have not been ob- 

 served upon one and the same individual, but by 

 comparing many individuals of the same kind in 

 their various stages of growth. 



Now what are the changes which take place in 

 the further growth of the star-fishes, and of the 

 sea-urchins ? In the star-fishes, as they are fig* 

 ured here (Plate IV) the number of calcareous 

 plates is still very small. Only those which sur- 

 round the mouth, five in number, have acquired a 

 certain size, with those which protect the extremi- 

 ty of the rays, also five in number, and which are 

 also considerably developed ; small ones in addi- 

 tion, are successively forming in pairs in the in- 

 tervening spaces, (Plate IV, fig. C) between which 

 suckers come out. Gradually more and more are 

 developed, the animal pushing in this way the 

 primitive and terminal plates of the rays further 

 outwards, 



It is therefore by the further intercalation of new 

 plates between the terminal and the oral ones, that 

 the rays are elongated, and they may grow to a 

 very prominent form, as we have here, [Plate XIII, 

 fig. A.] By varying proportions of their plates, 

 the rays, however, may grow to form very differ- 

 ent outlines of these animals, as may be ascertain- 

 ed by comparing their arrangement in different 

 genera of the family. 



In the Echini the growth is more difficult 

 to understand. How is it that the circular 

 body can grow larger by the addition of new 

 plates ? In order to understand that, let me men- 

 tion the facts which I have been able to trace with 

 reference to their growth, If we have here an 

 Echinus of a certain size, we will observe that its 

 plates are arranged in two different kinds of rows, 

 [Plate XIII, fig. C.] You see these rows alternate 

 with each other, (Fig. B), and here again, (Fig. A), 

 very narrow rows, alternating with much broader 

 ones. The vertical rows of plates leave a circular 

 hole above, which is closed by plates of another 

 character. And below another hole s which is closed 

 by plates of another character, but the sphere it- 

 self consists of plates, of two characters, narrower 

 ones, which have holes in themselves, and broader 

 ones,which have no holes, but upon which we ob- 

 serve more prominent tubercles, to which the 

 spines are attached moveable spines, of wbick 



