EEPORT ON THE ECHINOIDEA. 11 



actinal plastron of which, perhaps, the most embryonic type is that of the Ananchytidaj, 

 in which the plates comprising it are more or less hexagonal or pointed, extending beyond 

 the median interambulacral line. 



Very important changes also take place in the actinostomic plate of the odd interam- 

 bulacral area, which becomes, as is well known, the lip terminating the more or less keeled 

 actinal plastron, which thus becomes a kind of plough to shovel into the actinostome the 

 sand or mud in which such forms live. It is upon this plastron also that we find developed 

 the flat paddle-shaped spines so characteristic of the Spatangoids of the present day. 

 This lip is but little prominent in the older Spatangoids, it has no prominence in the 

 Collyritidse; in fact, we might call that form of actinostome the palseostome. It is still 

 represented at the present day in PalcBOStoma, and the young of all Spatangoids have such 

 an actinostome of a more or less pentagonal form. In the Clypeastridse, the Galeritidse, 

 and the Cassidulidfe we have modifications of the actinostome which subserve more or less 

 the same purpose as the strongly labiate actinostome of the majority of recent Spatan- 

 goids ; but in these types the actinostomic plate of the odd interambulacrum is not 

 specialised, and we can trace its growth very satisfactorily from the CoHyritidse to the 

 Hemiasteridse, the Spatangidae, and the Schizasteridse of the present day, while we find 

 in the Pourtalesise and in Palceostoma the persistence of the ancient actinostome, com- 

 bined in the latter with many recent structural features of the Spatangina. 



I have already, while speaking of the Pourtalesias, called attention to the mode of 

 development of the anal snout of that group from the episternum of the Spatangoids 

 proper. 



Anal System. 



In the structure of the plates of the anal system we have from embryological data a 

 clear explanation of the function of the anal plate of the Salenice. This anal plate was, 

 if we can trust the figures of Schmidt, perhaps abeady developed even in Bothriocidciris, 

 and may have existed in other Palseechinidae, though it may be difficult in the plates 

 covering the anal system of that group so easily to recognise the original anal plate as we 

 can do in many of the recent Triplechinidse. The figures of Bailey, and of Meek and 

 Worthen, and a specimen of Lepidesthes which I have had occasion to examine, would 

 seem to indicate a splitting up of the central plate into a great many smaller plates, more 

 according to the mode in which it takes place in the Arbaciadce. The excentric position 

 of the anal opening is also clearly shown to be due merely to the development of new plates 

 along one edge, while where the anal opening is formed symmetrically, we have the anal 

 system as in the Arbaciadse covered by a few large plates. 



The transition of the opening of the anal system to the area within the genital ring,^ 

 and into the odd interambulacral region, as we find it in all Spatangoids, is not, however, 



