i6 DISCOVERY REPORTS 



and others are associated with the teeth and other grinding parts. In the Nebaliacea the dorsal ridge 

 always forms part of the grinding organ of the gastric mill and at least one dorsal dilator is present 

 in all species. 



Other muscles are developed in association with special parts or functions ; for example, the muscles 

 {t.p.ni. Fig. 6 B) which move the trilobed process of NebalieUa back to the vertical position after the 

 food has passed, and those (v.m. and c.p.m. Fig. 6 A) which cause the elliptical motion of the tip of 

 the cardio-pyloric valve in Nebalia. 



The numerous small muscles which are not attached to particular structures in the gut probably 

 function in steadying the whole organ in relation to the other parts of the body. 



Thus, though the plan of the musculature is simple and constant, the changes may be associated 

 with the structure of the chitinous parts, and these in turn may be correlated with the habits of the 

 species concerned. 



Nebalia and NebalieUa both live where the bottom deposits are muddy, but observations of the 

 former, when living, show that it lies most of the time above the mud just beneath or amongst larger 

 debris of pieces of seaweed, shells and stones. The particles on which it feeds are thus the small ones 

 in suspension in this zone. NebalieUa, on the other hand, appears to be a true mud dweller. The 

 specializations of the eyes, rostrum, and antennae are adaptations to burrowing, and the food particles 

 found amongst the limbs and in the gut indicate that it feeds indiscriminately on the mud. Many of 

 the particles are too large to stay long in suspension. Therefore it must either allow some to pass 

 into the carapace chamber as it burrows or kicks up the mud and then filter rapidly before it settles 

 (as on occasions does Chirocephalus). 



In this mud there is a much higher percentage by volume of silica and other inorganic matter than 

 in the suspension of finer particles taken by Nebalia. This means that the material which NebalieUa 

 swallows has a lower food value, and there must be more of it. The mechanism, which is already 

 present in Nebalia, for rapid dealing with much food is elaborated and that for efficient grinding of 

 a little is reduced. The food is largely retained in the through passage of the gut and not passed into 

 the diverticula. In this way the indigestible particles are passed on rapidly, and such nutriment as 

 can be easily extracted by the digestive enzymes is obtained. In morphological association with this, 

 the openings and lumina of the digestive glands, are reduced and the lumen of the intestine increased, 

 and in place of the filter allowing the passage of selected finer particles into the glands there is a 

 mechanism whereby they are almost all excluded and passed straight on down the intestine. 



Thus the differences between the structure of the gut of Nebalia and that of NebalieUa may be 

 definitely associated with the habits of these animals and the food thus made available. 



Nebaliopsis, which has so far been found only at great depths in the open ocean, is in very different 

 surroundings from the bottom-living forms. It is only to be expected that adaptations to these 

 conditions would cause specializations, such as are found both in the external and in the internal 

 structures. The form of the gut may be correlated with the difference in food.^ 



Filterable particles are much scarcer in this zone, and, as has already been shown, the mechanism 

 whereby much useless material is passed rapidly through the gut has disappeared. A special method 

 for dealing with an entirely different type of food has been developed. This food is almost certainly 

 eggs, and in adaptation to the periodic abundance and scarcity of these the large digestive sac has 

 been developed as a store chamber and the lumen of the intestine has been reduced to insure that 



1 Since the above was written my attention has been drawn to a description by T. J. Evans (Q.J. M.S. 1922, lxvi N.S. p. 439) 

 of Calma glancoides, an Aeolidiomorph Nudibranch which feeds exclusively on ' the eggs and embryos of the smaller shore 

 fishes'. The amazing similarity between the adaptations of this mollusc to an egg diet and the specialized structure of 

 Nebaliopsis forms additional evidence that the latter also feeds on eggs. This is a remarkable case of parallel adaptive evolution 

 in two animals widely separated in phylogeny, habits and habitats and it is hoped to elaborate the comparison elsewhere. 



