and Embryological Development. 359 



Clypeastroids is as yet prominently developed. But rapidly, 

 with increasing size, the number of primary tubercles increases, 

 the spines lose their disproportionate size, the pores of the 

 abactinal region become crowded and elongate, and a rudi- 

 mentary petal is formed. The test becomes more flattened, 

 the coronal plates increase in number, and it would be impos- 

 sible to recognize in the young Echitwct/amus, for instance, 

 the adult of the Cidaris-like or Echinometra-like stages of 

 the Sea-urchin, had we not traced them step by step. Most 

 interesting also is it to follow the migrations of the anal 

 system, which, to a certain extent, may be said to retain the 

 embryonic features of the earlier stages of all Echinoderm 

 embryos, in being placed in more or less close proximity to 

 the actinostome. What has taken place in the growth of the 

 young Echuwci/amus is practically repeated for all the families 

 of Clypeastroids : a young Echinarachniics, or Mellita, or 

 Encope, or a Clypeaster proper resembles at first more an 

 Echinometra than a (Jlypeastroid ; they all have simple 

 poriferous zones and spines and tubercles out of all propor- 

 tion to the size of the test *. 



When we come to the development of the Spatangoids 

 we find their younger stages also differing greatly from the 

 adult. Among the Nucleolidre, for instance, the young stages 

 have as yet no petals, but only simple rectilinear poriferous 

 zones. They are elliptical with a high test, with a single large 

 primary tubercle for each plate, and a simple elliptical actino- 

 stome, without any trace of the typical bourrelets and phyl- 

 lodes so characteristic of this family. Very early, however, 

 this condition of things is changed, the test soon becomes 

 more flattened, the petals begin to form as they do in the 

 Clypeastroids, and we can soon trace the rudiments of the 

 peculiar bourrelets characteristic of the family, accompanied 

 by a rapid increase in the number of tubercles and in that of 

 the coronal plates. 



Among the iSpatangidse some are remarkable in their adult 

 condition for their labiate actinostome, for the great develop- 

 ment of the petals, for the presence of fascioles surrounding 

 certain definite areas, for the small size of the tubercles, the 

 general uniformity in the spines of the test, and the speciali- 

 zation of their anterior and posterior regions. On examining 

 the young stages of this group of Spatangoids, not one of 

 these structural features is as yet developed. The actino- 

 stome is simple, the poriferous zone has the same simple 



* Among the Clypeastroids I have examined the young of Echino- 

 cyamus, Fibularia, Meliita, Laganum, Echinarachnius, Encope, Clypeaster, 

 and Echinanthus. 



