730 EMBRYOLOGY. 



that of Echinocyanuis. This pluteus {Fig. 04) is remarkable for its rounded 

 extremity and the compact space filled by the oesophagus and stomach. The 

 only other Echinoid pluteus known to me which may be a Clypeastroid em- 

 bryo is that which I have figured in Fig. 65- It has, like the pluteus of 

 Echinocyanuis, a rounded lower extremity, the same compact alimentary 

 system, and arms arranged in pairs. Fig. 65 is most probably the pluteus of 

 Echinarachnius. It is quite common at Newport, R. I. ; as the embryos of 

 Strong3'locentrotus and of Arbacia, the two other Echini common in Nar- 

 ragansett Bay, are well known, it can only he the pluteus of Echinarachnius, 

 which abounds along I he sandy shores of the bay. It has a certain analogy 

 to the so-called Echinocyamus pluteus; at any rate, it is a remarkably dis- 

 tinct type from either the Spatango'id or the regular Echinoid pluteus. 



Among the Desmosticha the embryos of several .species of Echinus and 

 Stroneylocentrotus have been figured, showing striking differences. How 

 far these differences are of systematic value cannot he stated, as the embryos 

 known represent only a lew of the families of Echini. No embryos of Cida- 

 ridae or Diadematidae have as yef been observed. The pluteus of the Arba- 

 ciadae is well known from one of the most complete Memoirs of Miiller. A 

 few additional points on the development of Arbacia have been observed by 

 Metschnikoff. The pluteus of our Arbacia punctulata, which is tolerably 

 common at Newport. R. I. 1 Fig. 66), differs somewhat from the pluteus of 

 Arbacia figured by .Miiller, but not more than the pluteus of closely allied 

 species would he likely to differ. From this pluteus {Fig. 66) the young 

 Arbacia figured on page 734 was developed in about three weeks. The 

 young Arbacia was most remarkable, differing \cr\ strikingly from the young 

 Arbacia figured by Miiller, which lie also raised from the pluteus. The only 

 other type of pluteus known is a remarkable one. closely allied to Arbacia. 

 which has been figured by Miiller ( Fig. 67). From what has been said we 

 have evidently, as far as known, quite marked types of embryos correspond- 

 ing more or less to the several subordinal aud family divisions recognized 

 among Echini. 



