ECHINOIDEA. II. 



99 



It is a remarkable faet that iii souie of the small specimens only oue tubefoot is developed in 

 the piates I. a. i and V. b. i ; the posterior tubefoot in these piates must then develop later on. l^'rom 

 the specimen of 3'"'" (Fit;. 16) it appears that the first tubefoot to develop is the inner oue of the 



piates I. a. I, II. a. i, III. b. i then follows that of the piates I. b. i, II. b. i, III. a. i and la.stly 



the outer tube-foot of the piates 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 oue speciuieu I have fouud both tubefeet developed in the plate 

 I. a. I, only oue in V. b. i. — vSouie of the piates in the outer series of the bivial ambulacra may want 

 pores totally; this uiay hold good also for one or two of the piates 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 accordauce with that described 

 and figured in the best possible way by Loven for the more advanced stages studied l)v hiui. It is 

 extremely important to learn, how it is in the fullgrowu 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 Hgured by G au thi er" 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 piates. 

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

 genital papillæ. A few madreporic pores are fouud also in the left posterior 



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



Hcmiastcr expergitits. 

 The spines of the auterior end of the test are somewhat spearshaped, -mm ;„ length. 



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



Those on the posterior end of the test are more spoonshajDed, 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-, the widened part being sometimes almost quite 



hyaline, almost withont 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 t)-pical form. The spines withiu the fasciole are 



more or less spoou-shaped; tho.se along the auterior ambulacrnm increase in length towards the apical 



system, the uppermost being the longest, reaching even beyoud the fasciole behiud (not widened in 



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



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



I Recherches sur l'appareil apical dans quelques espéces d'Echinides apparteiiant au geure Heniiaster>. .\ssoc. Franc, 

 pour l'avancenient des Sciences. 1SS6. It is especially to be reuiarked that in a single species, Hemiaslcr batncnsis, 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 développemeut de l'Echinospatangus neocomiensis d'Orbigny. BuU. Soc. Yonne. 

 1S89. p. II. Note: De Loriol. Notes pour sen^ir å l'étude des Echiuodernies. VI. Rev. Suisse de Zool. V. 1897. p. 175; 

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



- In the tBlake -Echini p. 67 Professor Agassiz says of these spines in the young II. Mentsi: „The outer shcath of 

 calcareous rods becomes soHdified as thin lamellæ, 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 shatt of the radioles; in the shorter 



radioles of the actinal plastron the lamellæ all develop into this spoou-shaped extremity.. — Only two of the lamellæ develop 

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



13* 



