ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 451 



power of 500, the plexus will be found to consist of a large number of 

 fibrils, and some of the principal bundles will be seen passing towards 

 the si)ines and adjacent pediccllarife. The fibrils of which this jjlexus 

 is formed are identical with those of the tentacular and ambulacral 

 nerves, and each is continuous with the fibre from the ambulacral 

 nerve which emerges from one of the tentacular pores ; the plexus 

 lies between the external epithelium and a layer of connective tissue 

 which sends off a number of connective bands through tlie meshes of 

 the nervous plexus to support the epithelium. At the base of each spine 

 there is a relatively well developed nervous ring. The cellular 

 elements of the plexus are very difficult to detect in the plexus, but 

 they are very numerous and easy to see in the nerve-ring ; the author 

 does not, however, agree with Mr. Romanes in his description of 

 these elements. M. Prouho has also been able to make out a nervous 

 genital ring, which connects the five genital glands with one another, 

 and, by means of the five ambulacral trunks, with the peribuccal 

 nervous pentagon. 



New Echinothurid and its Poison-apparatus.* — Herrcn C. F. 

 and P. B. Sarasin have found in the Bay of Trincomalee a new 

 Echinotluarid, for which they propose the name of Cyanosoma urens. 



While presenting many resemblances in coloration to AstJienosoma 

 varium, it presents generic differences in the structure of its skeleton, 

 but they have not been able to study the descriptions given by A. 

 Agassiz of the ' Challenger ' Echinoids. If one seizes a specimen 

 one immediately feels a number of extremely painful stings, like those 

 of a bee, but the sense of pain is soon relieved. The organs which 

 effect this are spines in the dermal covering ; when best developed 

 they have the appearance of small blue stalked capitula, which are 

 traversed by a fine spine, the tip of which projects a little or not at 

 all beyond the soft parts ; the upper end of the spine is continued 

 into a pretty wide and strong sack of connective tissue, which is 

 continued as a solid lamella through the spine ; the end of the spine 

 inclosed in the sack is thus completely shut off from the more basal 

 portion. The head itself which surrounds the poison-bag consists 

 essentially of muscular fibres together with connective-tissue and 

 jjigment-cells ; the fibres are generally so arranged as to run parallel 

 to the siarface of the head, and are attached at one end to the poison- 

 bag, and at the other to the spine below. 



The mechanism appears to be of the following nature : when an 

 object presses on the top of the spine, the musculature of the head 

 contracts ; this breaks the poison-bag inferiorly ; and the greater part of 

 the tip of the spine becomes free ; at the same time the secretion con- 

 tained in the bag is pressed into the spine through the large orifices 

 at its base, and so make their way into the wound which the spine 

 has made. 



The other spines in the tegumentary coverings are formed in the 

 Biime way, but the mechanism is so far modified that the poison-bag 

 is not compressed by muscular force, but by the pressure of the 



* Zool. Aiizeig., ix. (188G) pp. SO-2. 



