30 THE VOi'AGE OF H.M.S. CHALLENGER. 



in the middle liue (PL V. fig. 3, vc), and the fibres to the ventral wall of the shield radiate 

 from the basement-tissue. Dorsally several less developed plumes also make their 

 appearance (PI. VI. fig. 4), and the pale (nervous) area lies under the hypoderm. Next 

 the two nuchal chambers take the place of the former, and the hypoderm over the median 

 nerve-area increases much in thickness, the whole forming a somewhat triangular region 

 between the bases of the plumes, which are thus carried outwards. None of these plumes 

 are so well developed as the first pair. While the nerve-area retains a large size, with the 

 two chambers and the fan -like fibres ventrally (PI. VI. fig. 5), the knife now severs the 

 anus, which at this stage lies close behind the massive dorsal hypoderm of the nerve-area. 

 Then the glandular wall of the buccal chamber and the post-oral lamella appear, while 

 the body-cavity on one side of the rectum presents a granular mass (PI. VII. fig. 4, ov), 

 the rudiment of the ovary. The great buccal disk becomes much broader as well as 

 thinner, and is solid, while a central chamber appears in the post-oral lamella. The 

 body-cavity is much better marked in the young forms than in the adults, and 

 just behind the mouth (PI. VII. fig. 5, r, vt, oe) has in section a symmetrical arrangement 

 of the gullet, stomach and intestine, the body-wall showing the basement-layer beneath 

 the hypoderm. The pharjTix, gullet and stomach are rounded or ovoid in transverse 

 section, but the intestine is triangular. The longitudinal muscular fibres appear along 

 the ventral wall immediately behind the mouth, and soon form the marked pattern so 

 characteristic of the region (PI. V. fig. 4, co). The body-cavity thus shows from above 

 downwards the comj)aratively small intestine, now rounded, and the massive glandular 

 wall of the stomach in the middle, both being surrounded by a firm investment which 

 leaves what may be called the keel of the stomach to be attached to a pointed incurvation 

 of the ventral wall. On each side of the latter is the thick central mass of the longi- 

 tudinal muscles, which externally also present another increase before being lost on the 

 body- wall. A considerable perivisceral cavity exists on each side of the digestive organs. 

 Behind the foregoing the body-wall becomes thicker and the central chamber less, 

 especially as the stomach ends in the intestine. The ventral muscles have considerably 

 increased in bulk, and the double inflection for the longitudinal muscles on each side of 

 the median line more marked. At the curvature of the alimentary system in the Ijody- 

 cavity, the pedicle proper commences, the external wall having a proportionally thicker 

 coating of hypoderm than the body-proper, and supported internally by the basement- 

 layer, which forms the W-shaped 2:)attern inferiorly with a secondar}^ curve on each outer 

 leg. The ventral half of the space is filled with the muscular fibres, while the rest is 

 split into two divisions by the median septum continued from the alimentary canal. The 

 sides of the septum and the inner surface of the wall are covered with fibres. The 

 buccal shield extends backwards to this region in the form of a broad, thin lamella, having 

 a, median fine of basement-tissue separating the ventral and dorsal layers of hypoderm, 

 and so little has the pedicle increased in length, that in some cases the free posterior 



