WYVILLE THOMSON, ON SYNAPTA INH/ERENS. 133 



The body is cylindrical and vermiform ; a full grown speci- 

 men, moderately distended, is about a foot in length, by five 

 eighths of an inch in diameter. 



Small specimens, from clear sand, have the integuments 

 very transparent and nearly colourless ; but large specimens, 

 from low water-mark on the mud banks, where they specially 

 abound, are of a pale-reddish brown. The mouth is un- 

 armed, and is placed in the centre of a disc of circular mus- 

 cular fibres, which forms the anterior extremity. The mouth 

 is surrounded by a circle of twelve pinnated tentacles, five 

 digitations on either side and on the oral surface of each 

 tentacle eight minute sucking papillae are arranged in a 

 double row. The body consists of an external wall, bound- 

 ing a cavity filled with sea water, in which the organs of 

 assimilation and of reproduction float freely. Three distinct 

 elements enter into the structure of the external wall. The 

 outer layer is of considerable thickness, transparent, and with 

 a seemingly structureless basis. 



Imbedded in the structureless sarcode are groups of thread 

 cells ; scattered masses of the carmine pigment-cells, to Avhich 

 the colour of the surface is due ; minute endoplasts, slightly 

 blue by transmitted light; and the remarkable calcareous 

 plates of various forms, which are so highly characteristic of 

 this group. These latter consist, in the present species, of 

 the " anchors'^ and ^' anchor plates,'^ which have been so 

 frequently described, and whose development we shall discuss 

 hereafter; of thinly scattered miliary granules ; and of peculiar 

 irregular plates, like skeleton keys, or more like old Celtic 

 " fibulse,^^ grouped in masses near the bases of the tentacles, 

 and scattered more sparingly over their surfaces. The entire 

 outer surface of the body is roughened by innumerable low 

 tubercles, at whose apex the carmine pigment-cells and the 

 thread-cells are specially accumulated. The outer layer 

 passes into a firm sheet of delicate felted elastic fibres, to 

 which the consistency of the wall of the body is chiefly due ; 

 and this layer is lined by a continuous membrane, formed of 

 delicate transparent, transverse muscular fibres, "005 mm. in 

 diameter. Powerful rythmical contractions of this coat are 

 perpetually passing along the animal, contracting its body at 

 intervals to half its normal diameter, while between the con- 

 tracted portions the integuments are distended with sea-water, 

 and usually rendered so transparent as to show clearly the vis- 

 cera within. This inner muscular layer, and indeed the whole 

 of the interior of the visceral cavity, is as it were varnished 

 with a thin coating of transparent, structureless sarcode. 



