6 ON THE ECHINODERMATA OF THE 



reason of extreme crowding. Owing to the densely tessellated and compact nature of 

 the integument, the sucker-feet are incapable of being perfectly retracted ; and this 

 circumstance, together with the size and general habit of the animal, affords a striking 

 superficial feature, which readily characterizes the species amongst its Arctic con- 

 geners. 



The cuticle, although thin, is very strong, being indurated with calcareous plates, 

 of which two distinct layers are present ; in each of these the plates are quite different 

 both in size and form, and are so closely packed as to imbricate upon one another, 

 whilst the whole Holothuroid is rough and scabrous to the touch, in consequence of 

 a small spiculate elevation which rises from the centre of each of the plates that form 

 the superficial layer. 



The tentacles are ten in number, two being much smaller than the rest. They are 

 of delicate habit, and redivided into many branches, which are fine and thin. 



The mouth-ring is elongate, and resembles in a marked degree the form found in 

 Thyone. It is composed of ten plates, each of which is made up of two elementary 

 pieces, as may be well seen in the mouth-apparatus of a young specimen (Plate I, Fig. 4). 

 The radial elements are produced upward into a fine wedge-shaped peak, and down- 

 ward into two long, thin prolongations, which extend to the base of the apparatus, the 

 cleft formed between them being wide and well arched above. The alternating inter- 

 radial elements are somewhat wedge-shaped, equal in size to the upper portion of the 

 radial processes, and extend upward to the same height as these. The angle of 

 the plates, which abuts against the neighbouring radial member, is somewhat trun- 

 cate, and the suture between the two primary pieces slightly hollowed out in the 

 middle. 



The Polian vesicle of a young individual is represented in Fig. 4, in which 

 it is seen as a very distended sac, emanating midway upon a fine short tube. 



According to Dr. Liitken (I. c.) the respiratory organs in the adult animal cleave 

 immediately into four rays, one small and one larger on each side. They are but feebly 

 branched ; and although becoming generally thicker towards their extremities, it is only 

 here and there that any thing like a vesicle can be detected. 



Eespecting the anatomy of the alimentary canal, it may be noted that the oesophagus 

 is moderately developed, with its outer wall villate or densely covered with minute 

 papilla?, and that the intestine is large and much convoluted. 



The muscular system is somewhat small for a Cucumaria. The m. longitudinales 

 are thin and very narrow, and the m. transversales similarly slight. The m. retractores, 

 however, are strong and by far the most powerful bands of muscle in the animal ; they 

 are attached to the central part of the wedge-shaped upper portion of the radial member 

 of the calcareous ring, and join the longitudinal band of muscles midway between 

 the extremities. When the mouth-apparatus is retracted and these muscles are relieved 

 from tension, they appear very broad in comparison with the other bands, and are, 

 besides, much swollen out in their middle portion. The m. extensores are very fine 

 indeed and double, and are affixed to the radial pieces of the ring a little above the 

 attachment of the retractor muscles. 



