138 THE APODOUS HOLOTHURIANS 



The external epithelium is thickest on the digits, where it may be more than 

 twice as thick as it is on the body; it contains both nerve and gland cells. Ten- 

 tacle ampullae are commonly, but not always, present. 



Sense-organs. — The only structures in the Molpadiidsc which can be called 

 sense-organs are the tentacles, which are undoubtedly important organs of 

 touch, except possibly in Ceraplectana. The immediate seat of tactile sensa- 

 tions is in sensory nerve cells, which are scattered all through the covering 

 epithelium of the body. They are considerably elongated, and the inner end 

 is probably directly continuous with a nerve fiber; the outer end, however, is 

 not ciliated or peculiar in any way. They are most abundant in the digits, where 

 they are collected into groups forming very simple and primitive "sense- 

 buds" or touch-papillae. 



Nervous system. — The nervous system of a Molpadid is not essentially dif- 

 ferent from that of any other holothurian. It consists of the customary cir- 

 cumoral ring, flattened antero-posteriorly so as to be from two to five times as 

 wide as deep, five radial longitudinal nerve trunks, 15 tentacle nerves, and 10 

 bucco-pharyngeal nerves. The circumoral ring is about one-fourth of a mm. in 

 width and one-twentieth to one-tenth in thickness, and lies beneath a connective 

 tissue layer immediately internal to the base of the tentacles. On its inner face 

 there is a deep, narrow furrow. It consists of delicate fibers interspersed with 

 numerous ganglion cells, which are much more abundant than in the radial 

 nerves ; the ring is covered on its anterior and inner sides by an evident epithe- 

 lial layer, the nuclei of which are small and deep-staining and apparently are 

 associated with a non-nervoiis supporting tissue, fibers of which run among the 

 nerve fibers but across the nerves. The radial nerves consist of two parts, a 

 thick outer band, crescentic in cross-section, and a thin, inner band, closely ap- 

 pressed to the outer, separated only by a thin layer of connective tissue. This 

 inner band shows a median longitudinal furrow throughout most of its length, 

 and anteriorly divides into two branches, which subdivide and innervate the 

 neighboring muscles, both longitudinal and transverse, and thus disappears ; pos- 

 teriorly it terminates in a slight enlargement. The outer band runs backward 

 to the termination of the radius beside the cloacal opening. The muscles, both 

 transverse and longitudinal, are apparenfly innervated by branches from the inner 

 band, while the outer band chiefly supplies the integument. Both bands send 

 nerves to the rudimentary pedicels at the ends of the ambulacra. The histolog- 

 ical structure of the bands is similar to that of the circumoral ring. The ten- 

 tacle nerves arise from the posterior outer portion of the circumoral ring and 

 run upward on the inner face of the tentacles, forming a wide sheet, thickest 

 mesially, near its base, and diminishing in width and thickness as it passes up- 

 ward. Its histological structure is similar to that of the radial nerves. It gives 

 off solid branches of fibers running directly to the touch-papilhx} and isolated 



