ECHINOIDEA. I. 



67 



stantiated in an indirect manner in tlie literature that the primar\- spines on the actinal side are 

 inclosed in a thick bag of skin, and it may be seen rather easier on tlie animals themselves when 

 they are fairly well preserved. — These bags of skin may possiby contain poison apparatus; at all 

 events the living Phormosomes are said to sting when touched, and there seems to be no poison bags 

 on the spines of tlie abactinal side. 



These skin-covered spines are of a more complicated structure than the spines of the abactinal 

 side; only at the base it may still be seen that they have originally been tubular as the other spines. 

 They end in a broad serrate point (PI. XII. Fig. 11). In transverse sections it is seen that they are 

 tubular in the lower part with projecting, hollowed ridges (PI. XL Fig. 7 b); towards the point these 

 ridges become much less conspicuous and quite irregular. At the same time the cavity is filled by 

 an irregular net of meshes of fine calcareous threads running parallel to the longitudinal axis of the 

 tube (PI. XI. P""ig. 10). The spines of the abactinal side, as is seen from the excellent figure by Wyv. 

 Thomson (PI. LXII. 3), are hollow tubes, very regularly perforated, and ending in a long, fine point. 

 Most frequently, however, the thorns are both fewer and more feeble than in this figure. Transverse 

 sections show that here no projecting longitudinal ridges are found (PI. XL F'ig. 7 a). The spines on 

 the peristome are covered in their whole length b}- a thick skin, but they have no bag-shaped 

 widening in the point. The spines themselves are constructed as the primary spines of the actinal 

 side, the only difference being that they are not widened at the point (PI. XII. Fig. 19). 



The expression of marginal fasciole ^ , used by A g a s s i z of the close-sitting small spines at 

 the ambitus (xBlake -Echini, p. 34) is to be avoided, at all events for the present. Agassiz, to be sure, 

 thinks that they take(s) almost the prominence of a fasciole, and are (is) interesting as .showing how 

 such a structure may exist in a rudimentar>- form in the Desmosticha > (Chall. Ech. p. 98). I do not 

 think that it recalls to any striking degree the fascicles of the Spatangids, and at all events we have 

 for the present no safety that they are homologous formations. The expression of marginal fringe* 

 used by Wyv. Thomson is therefore to be preferred, as it is quite without morphological pretensions. 



Wyv. Thomson (op. cit. p. 735) states that the tube feet are provided with a sucker with a 

 well-developed calcareous rosette of four or five pieces . This sucker I have not been able to find; 

 according to my examinations all the tube feet, as well actinal as abactinal, end in a point, without 

 sucking disk. The spicules, which are, as stated by Wyv. Thomson, irregular, larger or smaller 

 fenestrated plates, are conmionly arranged in 4 longitudinal series. This is especially distinct in the 

 lower part of the tube foot; towards the point the plates become larger and arcuate, and at last they 

 surround the foot as a mail. There is no great difference between the spicules of the tube feet of the 

 actinal and the abactinal sides; they are only more slightly developed in the latter (PL XL Fig. 25). 



In young specimens of Ph. placenta the peculiar feature is found in the tube feet of the abac- 

 tinal side that onh- the uppermost one of the three tube feet that correspond to each ambulacral plate, 

 is well developed, while the other two are quite rudimentary. The same fact may also be found in 

 large specimens, and it may at all events most frequently be seen that the uppermost one of each set 

 of three tube feet (the one belonging to the inner one of the two small secondary ambulacral plates) 

 is more developed than the others. In these rudimentary tube feet no spicules are developed; neither 



