ECHINOCYAMUS PUSILLUS. 53 



PL IX. fig. 107., two different kinds of spines have been developed, 

 i. e. long true spines tapering towards the free point, and short ones 

 with the end broadly truncated and provided with long needles. The for- 

 mer occupy a space round the animal between the upper and lower sur- 

 faces, while the later ones protrude also from this space but principally 

 from the dorsal region. Resides, the truncated spines are distinguished 

 by a covering of long cilia, in consequence of which they may be able 

 to perform functions different from those effected by the long spines. 



In connection with the explanation of the development of the 

 plates and spines, it seems suitable to give a short account of the origin 

 of the sphgeridia, detected and first described by Loven ^). According 

 to him several forms of the Clypeastridse, and among them Echinocyamus, 

 have only a single spherid in each ambulacrum. As far as my experiences 

 go this is a rule, but in young specimens two of these sense-organs 

 are not seldom met with in some of the ambulacra, though in such cases 

 the second probably is subjected to a retroactive developmental pro- 

 cess. Exactly as Lovén says in his report on Pourtalesia with regard to 

 Cassidulus, the spherids in Echinocyamus also are at first free and un- 

 covered, PI. VIII., fig. 106 and PL IX, fig. 108, but become subsequently 

 overgrown by a calcareous net-work. It ought to be kept in mind, that 

 they do not become visible all simultaneously, but that those of the 

 trivious ambulacra first arise, the frontal portion of the young being 

 thus clearly marked. The origin of the spherids is as follows, PL IX, 

 fig. 110 a — e. Like the spines, they also appear as exceedingly small 

 tetrahedrons, three angles of which become bipartite so as to form a 

 star with six arms while the fourth grows into a small central vertical 

 spine. This passes at a very early stage, when the plates and spines are 

 still small, incomplete and enclosed within the Pluteus. The point of 

 the central spine sends out three upwardly directed rods, which are curved 

 and have their ends converging. One of these small deposits measures 

 about 0,01 mm. in length, and is held upright by the star which serves 

 as a supporter. Then, the three rods send out near their point of origin 

 a process which grows downwards in order to coalesce with the arms 



1) Om Echinoideernas byggnad. — Öfversigt af K. Vetenskaps-Akademiens 

 Förhandlingar 1871. N:o 8. p. 1065—1068. — Etudes sur les Échinoïdées. — K. Sv. 

 Vetenskaps-Akademiens Handlingar. Vol. XI. N:o 7. With 53 plates. Stockholm 

 1874 p. 3 — 11. — On Pom'talesia. — K. Sv. Vetenskaps-Akademiens Handlingar. 

 — Vol. XIX. Nio 7. With 21 plates. Stockholm 1883. p. 39. 



