REPORT ON THE MYZOSTOMIDA. 
77 
(fig. 6), the cuticle becomes considerably thinner. Below the cuticle is a layer of poly- 
gonal epidermic cells from , 06- , 01 mm. in breadth, and a muscular layer consisting of 
circular and longitudinal fibres. The circular fibres are extremely fine, and closely 
pressed together, whereas the longitudinal fibres are somewhat stronger, and separated 
by intervals. In specimens that have been but little squeezed, the integument is thrown 
into circular folds, sometimes interrupted by longitudinal folds, so that the surface is 
divided into ranks of large papillse, which were especially well seen in a specimen (fig. 5) 
viewed with its dorsal side in profile, and in which the papillae are of various sizes up to 
•017 mm. in height. This system of folds is continued on to the parapodia, which are 
provided throughout their length from base to tip with circular concentric folds ; the 
parapodia (fig. 2 p.) are not distinctly marked off from the body, and show no trace of any 
division into two portions, which is so typical of the Myzostomata ; they arise from the 
extreme margin of the ventral side. The third pair are the most strongly developed, the 
first and the last are the most feeble. The first pair are obtusely conical in form, and 
about ’2 mm. long in the largest specimens ; their breadth at the base is about *28 mm. 
Corresponding to the insignificant size of the parapodia, the muscles and hook-apparatus 
(fig. 6) are but slightly developed; the latter consists of a long, thin ('25 mm. long 
•006 mm. thick), straight hook (u.), with a short tip suddenly bent back, and of a slightly 
bowed, somewhat shorter but thicker supporting rod (m), which ends in a point, and 
shows no trace of any manubrial plate. The muscular apparatus is very simple ; it consists 
of a fan-like series of fibres ( m .), passing from the base of the parapodium and the integu- 
mental parts surrounding it to the base of the hook-apparatus, which they entirely 
envelope, serving no doubt as protractors. In no specimen, however, was the hook- 
apparatus stretched out, and indeed it was not generally even so prominent as in 
fig. 6, the extreme point alone being visible. Besides these protractor muscles are 
a series of radial fibres (m 2 ) passing to the end of the parapodium, and appearing to be 
joined partly to the supporting rod and partly to the integument at the end of the 
parapodium. 
A third group of muscles passes from the end of the supporting bristle to the base of 
the hooks (w 1 ), and corresponds to the musculi conjunctores of the typical Myzostomata. 1 
The most striking peculiarity in the condition of the parapodial muscles of Stelechojms is 
the absence of a musculus centralis, 2 which in Myzostoma is the most powerfully 
developed of all the muscles of the body, and combines with the musculi centrales 
of the other parapodia to form the central muscle-mass. There is nothing of the 
kind in Stelechopus, and the parapodial muscles are quite independent of each other. 
This arrangement is, without doubt, more primitive than the radial arrangement found in 
the typical Myzostomata, where the body is divided by radial septa into twelve sectors 
(ten for the parapodia, two for the pharynx and cloaca). Instead of this there are in 
1 Genus Myzostoma, pi. viii. fig. 1, cb. and cl. 2 Loc. cil., me. 
