,-,, ECHINOIDEA. I. 



his specimens had a diameter of 8 mm , he has not, of course, been able to give all the necessary 

 informations. To this is to be added that I must decidedly contest the correctness of several of the 

 most important statements of Agassiz. 



The form of the test is in specimens of a diameter of 3 mm as in a common Echinus, not flattened, 

 and the plates are not yet imbricated; already in specimens of a diameter of 5 mm the test is a 

 little flattened. In the smallest specimens the peristome is quite covered by the 10 large buccal plates; 

 only inside of these, nearest to the mouth, a few small, irregular plates are seen. All the 10 buccal 

 tube feet are well developed and of equal size; spines are not yet found on the buccal plates. In a 

 specimen of a diameter of 5 mm there are 5 spines on the buccal plates, one for each pair of tube feet; 

 here ambulacral plates have begun to appear on the buccal membrane outside of the buccal plates. 

 A specimen of a diameter of 7""" has 10 spines on the buccal plates alternating regularly with the 

 tube feet, so that spines and tube feet together form a regular circle; here also 5 spines have appeared 

 outside of the first circle, one opposite to each ambulacrum. According to Agassiz the buccal plates 

 in P/ioniiosoiiii! placenta should not differ in size from the other plates on the peristome, so that the 

 Echinid features of the actinostome did not seem to occur in this species. This is incorrect; in the 

 youngest stages the buccal plates are easily recognised by their size — but it is to be admitted that 

 this difference in size soon disappears, the other plates of the peristome reaching aboxit the same size. 

 Of these plates in the peristome Agassiz (op. cit. p. 32) says that they are developed ... independently 

 of the coronal plates; new plates forming on the distal surface of the actinostome, which are interca- 

 lated between the old plates and the coronal plates . This is absolutely incorrect; the plates of the 

 peristome are ambulacral plates displaced adorally (Loven); on a contrary supposition beginnings of 

 them and quite small plates must be found outermost in the peristome, but this is not the case — on 

 the contrary the outermost plates are the largest. In Challenger -Echinoidea p. 73 Agassiz also 

 says that these plates «are formed by becoming detached from the ambulacral zones . 



In the smallest of the specimens in hand there are as yet only ca. 7 pairs of tube feet, besides 

 the buccal ones. There is no distinct difference between the primary and the accessory ambulacral 

 plates; only in a specimen of a diameter of j mm the primary one begins to grow larger than the 

 others, and it carries now 1 — 2 tubercles, while the small ones have at most a small miliary tubercle. 

 In specimens of this size the areoles begin to be deepened, so that the difference between the actinal 

 and abactiual side is now already indicated. — Auriculae are already distinct in individuals of a diameter 

 of 6 mm , but are as yet only a pair of small processes, not connected above. The gills do not appear 

 till later; in individuals of a diameter of io mm they are not yet to be seen. A few triphyllous pedi- 

 cellaria;, of the same form as in the adult, and a few spha;ridise are already found in the smallest 

 specimens. - The apical area is in all essentials as in the youngest stage figured (PL IV. Fig. 2). The 

 periproct is, even in the smallest specimens, covered by a number of small, irregular plates, with no 

 larger plate between. So a central plate seems never to be found here. The genital plates join for 

 a long space, so that the ocular plates are widely separated from the periproct; these plates are much 

 lengthened, reach down quite to the middle of the test, and here the pore is placed, which, in accord- 

 ance with its morphological signification as the opening of the terminal feeler (the point of the 



