NEBALIOPSIS TYPICA 2i7 



specimens that are only slightly swollen show the muscles of the thorax torn away from the integument '. 

 Now, specimen F2 was sectioned down to the sagittal plain as I stated in my report (loc. cit. p. 204). 

 The sections are quite normal and are typical of deep-sea material. There is no such distortion of the 

 musculature as Linder predicts, or, in fact, of any other organ. The proof of this is that Miss Rowett 

 (1943) has used the series of sections to work out with conspicuous success the anatomy of the gut. 



Miss Rowett 's work was published a few months after the appearance of Dr Linder's paper, and 

 throws a completely new light on Nebaliopsis. Moreover, it supplies an obvious explanation of the 

 differences between our two specimens. She showed that the large opaque mass in my specimen F2, 

 which I tentatively suggested was the ovary (loc. cit. p. 203), was in fact an enormous sac-like diver- 

 ticulum of the mid-gut. It was filled with a homogeneous coagulum ; that is, with a mass of food material 

 in which there was no structure (Rowett, 1943, p. 15). From this, with admirable argument, she puts 

 forward the view that Nebaliopsis is an egg-sucker. She then shows that, quite apart from the apparatus 

 for fiker-feeding, this remarkable form is fully adapted in a variety of ways to this peculiar diet. In 

 a paper that is now in the press she has gone further and has pointed out the extraordinary correspon- 

 dence that occurs between the adaptation of Nebaliopsis and of the nudibranch Calma glaucoides which 

 is known to feed exclusively on a diet of eggs. 



The adaptation which is of importance in the present discussion is the enormous mid-gut sac. 

 Miss Rowett points out (loc. cit. p. 8) that this sac is 'without any convolution and with only a few 

 septa arising from its walls'. Therefore while it does not provide much extra surface for its digestion 

 it is admirably suited for a storage organ. Now eggs will certainly not always be present at the great 

 depths at which Nebaliopsis lives. During the breeding periods of neighbouring animals there will be 

 abundance, but in between whiles scarcity. However, it must be remembered that the neighbours of 

 Nebaliopsis are few and far between, so that even when eggs are present they will be patchy in their 

 distribution. A large storage organ is clearly an adaptation to this. It enables the animal to take a con- 

 siderable quantity of food on the infrequent occasions when it happens to encounter a patch. My 

 specimen F2 is clearly a specimen that had just had a meal and it became distended in the same natural 

 way in which, for example, a blood-sucking tick becomes bloated after a meal. Another parallel is to be 

 found in a deep-sea fish such as Chaismodiis niger (Murray and Hjort, 1912, p. 721, fig. 515) which has 

 an abdomen so distensible that it can accommodate a larger specimen of the same species. 



The musculature of the body appears to be arranged so as to allow the body to expand in the hinder 



trunk region. The abdominal region is a packed mass of muscles. The cephalic region contains all the 



musculature of the antennae and mouth parts and the muscles extend ventrally in association with the 



trunk limbs. They become less marked posteriorly in relation to the simple condition of the eighth 



trunk limbs. The dorsal and ventral longitudinal muscles are practically non-existent in the thoracic 



region. There is thus a large region of the body, the posterior and dorsal thoracic region, which is 



almost devoid of muscles. The integument over this region is very thin and flexible and it is here that, 



as my photographs show, the expansion takes place. u • u ^ 



To summarize, the enlarged appearance of the Discovery specimen F2 is due to the fact that it had 



just taken a meal. It is not in any way an unnatural distension. A comparable specimen in which the 



mid-gut digestive sac is empty is shown in Plate XV. This beautiful example occurs in a second small 



Discovery collection. I consider this specimen much more perfect than Linder's, for it shows the 



rostrum, eyes, and antennae in a normal position for one of the Nebaliacea. . , ^ , . 



The photograph which Linder published (loc. cit. Taf. I, fig. i) agrees fairly closely with the sketch 



published by Ohlin (loc. cit. fig. i) ; enough, in fact, to make it fairly certain that the sketch was made 



after fixation and not while the animal was swimming round. How the animal was preserved we do not 



know There is nothing to indicate the use of any special fixative, and so most likely it was placed in 



