ECHINOIDEA. II. 99 



It is a remarkable fact that in some of the small specimens only one tubefoot is developed in 

 the plates I. a. i and V. b. i; the posterior tubefoot in these plates must then develop later on. From 

 the specimen of 3""" (Fig. 16) it appears that the first tubefoot to develop is the inner one of the 



plates I. a. i, II. a. i, III. b. i then follows that of the plates I. b. i, II. b. i, III. a. i and lastly 



the outer tube-foot of the plates I. a. i, II. a. i, III. b. i It is also seen there that the latter 



appears first in the plate III. b. i. In one specimen I have found both tubefeet developed in the plate 

 I. a. i, only one in V. b. i. Some of the plates in the outer series of the bivial ambulacra may want 

 pores totally; this may hold good also for one or two of the plates of the inner series between the 

 proximal ones and those bearing the large subanal tubefeet (the 6th gth plate). 



The apical system of the youngest specimen (Fig. 17) is quite in accordance with that described 

 and figured in the best possible way by Loven for the more advanced stages studied by him. It is 

 extremely important to learn, how it is in the fullgrown specimens, as Loven holds its ethmophract 

 structure to be of very great systematic importance, a view not universally 

 accepted, the numerous transitional stages from an ethmophract to an ethmo- 

 lytic condition figured by Gauthier 1 tending especially to show this feature 

 to be of no primary systematic importance. As shown in Fig. 18 the apical 

 system of the largest specimen is as ethmophract as that of the smallest speci- 

 mens, the madreporic plate does not separate the posterior genital plates. 

 There are four genital pores, with well developed, up to more than 3""" long, 

 genital papillae. A few madreporic pores are found also in the left posterior 

 genital plate. The madreporic plate is often somewhat elevated. Fig- 18. Apical system of 



Hemiaster expergitus. 

 The spines of the anterior end of the test are somewhat spearshaped, 37imn in lengtll 



with coarsely serrate edge, in side-view curved and quite sharp. (PI. XV. Fig. 44). 



Those on the posterior end of the test are more spoonshaped, with smooth edge; the spines of the 

 sides of the test are intermediate in shape between these two forms. The spines of the actinal plastron 

 (PI. XV. Fig. 50) are much widened in the point 2 , the widened part being sometimes almost quite 

 hyaline, almost without any reticulate tissue in the middle; in others the reticulate tissue has a 

 greater extent, both kinds occurring together in the same specimen. It is worth noticing that in the 

 specimen of 3'" length these spines are already of the typical form. The spines within the fascicle are 

 more or less spoon-shaped; those along the anterior ambulacrum increase in length towards the apical 

 system, the uppermost being the longest, reaching even beyond the fascicle behind (not widened in 

 the point). The size of the tubercles is, of course, in accordance with this fact, as is seen in Loven's 

 Fig. 115. The small miliary spines are mainly of the same structure as the clavulse. (Comp. Agassiz 



1 Recherches sur 1'appareil apical dans quelques especes d'Echinides appartenant au genre Hemiaster. Assoc. Franc, 

 pour I'avancement lies Sciences. 1886. It is especially to be remarked that in a single species, Hemiaster batnensis, Gauthier 

 finds all stages represented from a typical ethmophract apical system in the young specimens to an ethmolytic in the large 

 specimens. (Comp. also: Lambert. Note sur le developpement de 1'Echinospatangus neocomiensis d'Orbigny. Bull. Soc. Yonne. 

 1889. p. n. Note; De Loriol. Notes pour servir a 1'etude des Echinodermes. VI. Rev. Suisse de Zool. V. 1897. p. 175; 

 A. Valette. Description de quelques Echinides nouveaux. Bull. Soc. Yonne. 1905. p. 44). 



2 In the Blake-Echini p. 67 Professor Agassiz says of these spines in the young H. Mentzi: The outer sheath of 

 calcareous rods becomes solidified as thin lamellae, forming in one case in the primary interambulacral spines of the anterior 



part of the test on the abactinal side, above the ambitus, a spearlike head to the shaft of the radicles ; in the shorter 



radicles of the actinal plastron the lamellae all develop into this spoon-shaped extremity. Only two of the lamellae develop 

 in this manner, the rest of them disappear on the lower part of the head. 



13* 



