EESPIRATORY MECHANISM OF CORYSTES CASSIVELAUNUS. 289 



II. SOME STEUCTUEAL PECULIAEITIES AVHICH APPEAE 

 TO HAVE PASSED UNNOTICED BY GAESTANG. 



On that part of the ventral body wall which forms the posterior part 

 of the dorsal wall of the prostomial chamber is a fairly prominent 

 calcified V-shaped ridge, the point of the V being directed backwards. 

 This ridge is formed by the projecting anterior edge of the epistomial 

 sternite, and is fringed with a row of fairly long hairs, which project 

 quite halfway across the entrance to each branchial chamber. 



Some of the mouth appendages of Corystes cassivelaumcs — first maxilla 

 and first maxillipede — show a rather curious modification. Comparing 

 these appendages with those corresponding to them in such a type as 

 Cancer (vide Pearson's " Cancer," L.M.B.C. Memoirs, No. xvi), the 

 enlargement of the endopodite is very noticeable. Corystes has the 

 endopodite lobe of each of these appendages (Figs. 4 and 5) specially 

 produced and fringed with hairs of a fairly complex type (Fig. 7). 



The mandible of each side is placed with its " apophysis " pointing 

 obliquely backwards, and forming a very prominent ridge projecting 

 towards the ventral side of the animal. Across this ridge lie tlie 

 specially enlarged endopodite lobes of appendages iv and vi, and over 

 it they are turned up in a dorsal direction, so that their hairy fringes 

 project across the entrance to the branchial chamber proper, meeting 

 the fringe of hairs arising from the V-shaped ridge of the epistomial 

 sternite (cf. Figs. 1, 2 and 3). 



Garstang himself notes that the prostomial chamber is but imperfectly 

 floored ; it is noticeable that to its central part there is no floor what- 

 ever, even a hairy one ; and even around its sides the hairs which 

 spring from different parts and converge to the centre neither interlock 

 to any great degree, nor have sufficient individual complexity of struc- 

 ture to constitute anything like a barrier to restrain the in-current of 

 water. 



The simple structure of the hairs on the antennae should be noted 

 (Fig. 6) : there is complete absence of any kind of arrangement for fllter- 

 ing the water as it passes from the exterior into the antennal tube, and 

 thence into the prostomial chamber. The habit of Corystes cassivelaiinv.s 

 is to lie buried in sand ; the water directly above the surface must 

 necessarily contain some particles of sand or mud, and it is from this 

 by no means clear water that the supply for the respiratory stream is 

 drawn. The complete absence of any arrangement fur filtering the 

 water of the incoming current has already been noted, but the sieve- 

 like partition formed by the hairs projecting from the specialised 

 endopodites of appendages IV and VI (Fig. 7), together with those of 



