280 STRONGYLOCENTEOTUS DKUBACH1ENSIS. 



more elongate, though still serrate; the tentacles are more numerous; the 

 anal system is developed as a single large plate (PL X. f. g), the anus 

 opening to one side of it ; and we commence to trace distinct sutures 

 separating the coronal plates, new plates being added at the abactinal 

 pole round the anal system. 



In young specimens of somewhat less than one eighth of an inch in diam- 

 eter (3" ,m ■), having five : six ambulacra! plates (PL IX. f. 3), we find features 

 totally unlike those belonging to the genus Strongylocentrotus. The anal 

 system is closed by one large circular plate occupying the whole of the space 

 between the genital plates. A line passing through the madreporie body and 

 the anal plate does not pass through the symmetrical axis of the plate, but 

 at a considerable angle, showing (as is the case in all young Sea-urchins) that 

 we cannot use the anal plate, or opening of anus, as a guide for the determi- 

 nation of any one axis in Sea-urchins. So that the attempt made to define 

 the longitudinal axis by a line passing through the axis of symmetry of the 

 subanal plate docs not give us any fixed position. This attempt was only in- 

 troduced from taking the subanal plate of Salenia as the guide in fixing this 

 axis, but if the subanal plate of Salenia is only the homologue of the large 

 anal plate of young Echini ( PI J X. f. ./. >;, s), it is natural to find the madre- 

 I nic' body to one side of it, as we find in the young of several genera of 

 Echini. The symmetrical axis of the anal plate pointing either to the second 

 or third genital plate (counting from the madreporie body to the left), and 

 as I have shown in my Memoirs on the Embryology of Echinodernis that the 

 plane of the madreporie body was one connected with a fundamental point 

 of structure in the plan of the young Echinodenn. there is no anomaly pre- 

 sented by finding in Saleniae the madreporie body passing asymmetrically 

 through the subanal plate, that being, as is the case in all Echini, its natural 

 position. 



In these small specimens we find the pores arranged in vertical arcs of 

 four pairs for each ambulacral plate (PL IX. f. ',). There are two main 

 vertical rows of primary tubercles in the ambulacral and inteiambulacral 

 zones, nearly of the same size, surrounded by a few secondary tubercles 

 irregularly arranged ; the test is quite flat in proportion to the diameter, and, 

 when seen from below, we find the actinostome of extraordinary proportions 

 (PL IX. f. 4), occupying fully three quarters of the whole lower part; the 

 outline is scarcely indented; the points of the teeth are blunt, triangular; the 

 membrane is naked, being slightly stiffened by small granules round the at- 



