HOMOLOGY OF THE SKELETON. 



15 



" It is the Asteridea, Ophiuridea, and Crinoidea, known popularly as star-fislies/' 

 remarks Professor Huxley, " which depart least from this common plan, the ambulacral 

 and antambulacral regions being in all these about equally developed, and the arms in 

 most cases distinctly marked off from the body. But there are certain star- fishes which 

 are nearly pentagonal : suppose one of these, as Miiller suggests, to be elastic, so as to be 

 capable of being distended with air into a globular form; then the ambulacral region, 

 with its five ambulacra, would occupy the entire apical hemisphere. There is no 

 Echinoderm which exhibits this globular form, with equality of the ambulacral and 

 antambulacral regions ; but if we suppose the ambulacral region to increase at the expense 

 of the antambulacral, so that the latter eventually became reduced to a very small space 

 around the apex, the result would be the form of Echimis, or of Rolotlmria, in which the 

 ambulacral region greatly predominates, the arms disappear, and the ambulacra are, 

 consequently, entirely calycine. One moiety of the CystidecB are in the same prcdicaracyit ; 

 but other Cijstidea, such as Echino-encrhius, Frunocystites, Cryptocrvnus, present a 

 precisely opposite condition, the antambulacral region here extending into the close 

 vicinity of the mouth, and greatly predominating over the ambulacral region. In the 

 Blastoidea again, the antambulacral and ambulacral regions are more upon an equality, 

 but the body is sub-cylindrical or prismatic in shape ; otherwise they would offer a close 

 approximation to the hypothetical form, intermediate between an Echinus and a star-fish, 

 mentioned above." ^ 



Having thus reviewed the opinions advanced by different authors on the homology of 

 the skeleton of the Asteiiiad^ as compared with the test of the Echinid^e, it only re- 

 mains for me to state as briefly as possible the views on this subject which I have for nearly 

 thirty years been in the habit of teaching in my lectures on comparative anatomy. I 

 regard the valley in the centre of the under side of the rays in star-fishes, through which 

 the tubular retractile feet pass, as homologous to the ambulacral areas and poriferous zones 

 in the EcHiNiDiE ; the ossicula forming the sides and upper surface of the rays of the 

 star-fish as the homologues of the inter-ambulacral plates of the EcniNiDyE, greatly modi- 

 fied for a special function. In order to show the relation of these parts to each other, I 

 take a moderate-sized TJrader rubens, Lin., dead some hours, and quite flaccid, and dissect 

 out a circle of the integument in the centre of the upper surface of the disc, including 

 therein the madreporiform tubercle, vent, and genital pores ; the part thus removed will 

 represent the anal area. With a pair of scissors I then lay open the upper surface of all 

 the rays by a straight incision down the middle, from the circumference of the anal circle 

 to the extreme point of the ray, and, folding down the two flaps thus produced from each 

 ray into the inter-radial spaces, with a needle and thread sew the lateral flaps from the 

 adjoining rays together; when the whole of the flaps are thus united, I raise the border of 

 the flat disc and form the whole into a globular shape, taking care to make the extreme 

 points of the rays, with their eye-spots, touch the margin of the anal circle, which must be 



' ' Medical Times and Gazette,' new series, No. 332, p. 463. 



