26 BULLETIN OF THE 



The dorsocentral is hardly larger than a small calcareous nodule, and 

 the genitals and termiuals are but simple branched spicules. Not a 

 single plate of the actinal region of the body has yet appeared. It will 

 be seen that plates which have been regarded of importance in a discus- 

 sion of the affinities of the starfish with other Echinoderms are not even 

 present as a simple rudiment, when spines which few have yet consid- 

 ered of any great importance morphologically have begun to form, and 

 are well developed in many cases. Are we justified in thus neglecting 

 the spines, or have they no morphological meaning outside of a simple 

 classificatory interest 1 Tlie discussion of the meaning of the large size 

 of the first-formed spines will be taken up later. It is necessary now 

 for us to consider the size, arrangement, and distribution of these struc- 

 tures, their embryonic form, and their general mode of growth. 



The spines originate as trifid spicules, and in their early stages are not 

 to be distinguished from calcareous plates. The earliest spines (PI. I. 

 fig. 3, sp) to appear are those at the outer rim of the terminals of 

 the young starfish, peripherally to these plates. They are at lirst ten in 

 number, or two to each terminal, and by the time the notclies which 

 indicate the interbrachial regions of the future starfish arc incised, 

 the number of immature primary spines has increased to nine. Later, 

 when the number of ambulacral rafters lias grown to six (tliree pairs), 

 thei'e are sis very prominent spines on the outer l)order of the termi- 

 nals. According to A. Agassiz, these spines are more or less fan-shaped, 

 and recall those of certain Echinoids. The dorsocenti-al, for a long 

 time after its first apijearance, bears a single long and prominent spine. 

 This calcification is jointed to the centre of the dorsocentral on the 

 aboral side, and later other primary spines are added to the dorsocentral. 

 The genitals (basals) have at first three long, slender spines, which 

 originate while yet the starfisli is borne by the brachiolaria. The 

 spines of the medial dorsal plates of the arms are prominent and single 

 at first, each situated in the middle of the plate upon which it is carried. 

 The lateral or marginal plates, m, of the arms bear long, stout, single 

 spines. Each interambulacral plate, rtr7, has at first a single spine. No 

 spines were ever observed in the anibulaci'als, but the extremities of 

 the oral aiubulacrals in the interbrachii bear four spines, two of which 

 lie on the edge adjacent the mouth. Tlie so-called odontophorcs * were 



* From the position wiiicli they early occupy, it is not to be woiulered at tliat 

 spines are not developed on the first interbrachials or the odontopliorcs. They 

 are covered on the actinal side by the interbrachial ends of the oral ambulacrals, 

 so that spines could not be formed. 



