48 THE APODOUS HOLOTHUEIANS 



Body-wall. — The body-wall of the Synaptids is thin and generally translu- 

 cent, if not actually transparent. In Myriotrochus and some species of Lepto- 

 synapta and Chiridota it is so transparent that the inner organs are readily 

 seen through it. It consists of five parts (Plate VI, fig. 21), a cutis and a layer 

 of epithelial cells, which together make up the epidermis ; a layer of connective 

 tissue in which lie the calcareous bodies, pigment, etc., a layer of circular mus- 

 cles — in the radii the longitudinal muscles — and an inner epithelium, which 

 lines the body cavity. 



1. Epidermis. — The cutis is a thin, transparent, structureless layer, se- 

 creted by the epidermal cells, and exhibiting no special peculiarities. The epi- 

 thelial cells beneath are of three distinct sorts; ordinary, more or less elon- 

 gated, irregularly polygonal cells, which greatly predominate, but among which 

 are scattered sensory and gland cells, the former in clusters of several dozen, 

 forming the "sense-buds" to which reference has already been made. The 

 sensory cells are much more elongated than the ordinary epithelial cells, and 

 the inner end is drawn out into a fiber which connects directly with the small 

 ganglion lying underneath the sense-bud. Sometimes such sensory cells occvir 

 scattered singly among the ordinary epithelial cells. The gland cells are of two 

 kinds, but their number, size, distribution, and activity undoubtedly differ 

 much in different Sjoiaptids, according to the manner of life. The so-called 

 goblet glands are the more numerous, and occur in nearly all jiarts of the 

 epidermis, while the larger club glands are much less common and more widely 

 scattered. Both kinds of glands are most numerous and best developed in the 

 epidermis of the tentacles, especially on the outer side near the tip and on the 

 oiiter side of the digits, the surface of which is quite viscid in most Synaptids. 

 Whether the so-called "contractile rosettes" (see Becher : 06) are epidermal 

 structures does not seem to have been determined, but they probably lie deeper. 

 Very possibly they are identical with the larval glandular organs of Synap- 

 tnla hgdriformis referi'ed to on page 61. 



2. The connective tissue lager makes up as a rule more than three-fifths of 

 the body-wall, although tire proportion varies with the total thickness. It con- 

 sists of a mass of fibers, the outgrowing prolongations of spindle- or star-shaped 

 cells, imbedded in a clear, transpai-ent, nearly colorless ground-substance of 

 gelatinous consistency. Besides these connective-tissue cells, so-called ivander- 

 ing cells are abimdant, which are more or less oval in outline, but often pos- 

 sess pseudopodia as they are capable of amoeboid movements. In the connect- 

 ive-tissue layer are found two of the most important structures of the body- 

 wall, the pigment cells and the calcareous bodies. The pigment cells are some- 

 times more or less oval or spherical bodies, but are often irregular, much- 

 branched particles of coloring matter, most numerous in the outer part of the 

 body-wall just below the epidermis. They are often aggregated in particular 



