ENDOCYCLICA 541 



clefts, or indentations of the edge of the corona from which the 

 gills are extruded. Its most marked peculiarity, however, as 

 shown hy both Mortensen and Uexkiill, consists in the highly 

 developed character of its gemmiform pedicellariae, on the stalks 

 of which are situated glands. When the head with its poison- 

 glands is torn oft', the secretion of these stalk -glands can envelop an 

 enemy with a glutinous secretion, which impedes its movements. 

 The blades on a slight mechanical stimulus divaricate very widely 

 and become locked in this position, so that the enemy's body gets 

 in well within their reach. The muscles of the poison-glands 

 contract, but their ducts are bent by the act of opening, so that 

 the secretion cannot escape. The sense-organs have stiff hairs, 

 which penetrate the surface of the enemy and cause its juices to 

 exude and so stimulate the blades to close, and at the same time 

 permit the poison to be expelled. It will be remembered that 

 the gemmiform pedicellariae of Echinus open in response to a 

 chemical stimulus and close on a mechanical one being super- 

 added ; so that their responses are the direct opposite of what 

 occurs in Sphaer echinus. S. granulans, a Mediterranean species 

 with short red spines, just reaches the Channel Islands. 



Strongylocentrotus has shallow gill -clefts and gemmiform 

 pedicellariae, like those of Echinus, except that they have a 

 muscular stalk. In the British area it is represented by two 

 species, S. lividus, in which the primary spines are markedly 

 longer than the secondaries and are of a brownish purple colour, 

 and S. droebachiensis, in which the primaries are little longer 

 than the secondaries and are of a greenish brown colour. 

 S. lividus occurs abundantly in the Mediterranean, and reaches 

 the" English Channel and the west coast of Ireland. In the 

 last-named locality, where it is exposed to the full sweep of the 

 Atlantic, it is said to excavate holes for itself in the limestone 

 rocks, about ten inches in depth.^ ^S'. droebachiensis, which has 

 been recorded in the British area, chiefly from the west coast of 

 Scotland, is one of the most abundant members of the fauna of 

 the east coast of America. In the Gulf of St. Lawrence and in 

 the branches of the Bay of Fundy it is found in thousands, and 



1 Mr. E. W. L. Holt, Scientific Adviser to the Irish Board of Fisheries, casts doubt 

 (in litt.) on much of this supposed excavation. While disclaiming any novelty in 

 this observation, he points out that in many cases one side of the cavity is formed 

 by calcareous algae, and it seems as if the animal wanders into a crevice, in which 

 it is imprisoned by the growth of this plant. 



