CEUSTACEA 



101 



lined with a chitinous cuticle, and derived from a secondary 

 invagination of the ectoderm ; a mid-gut or enteron, represent- 

 ing alone the primitive enteron of the embryo and from which 

 the paired digestive gland originates ; and a hind-gut or 

 proctodeum practically the whole of the intestine developed 

 from an ingrowth of the ectoderm. 



The Stomodeum. The mouth is a v-shaped opening, a 

 vertical fold of the anterior lip, or labrum, projecting backwards 

 into the cavity. It is bounded at the sides by the mandibles. 



Cerebral ganglion 



Ant. 

 Ophthal. | 



Pericard. 

 Hi. sinus 



Dors. abd. Int. 



Nerve Sub- 

 cord neur. 



Green gland 



Mouth Stomach Dig. gland Sternal Testis 

 FIG. 48. Nephrops. Median longitudinal section of male. 



Tdson 



The red-coloured gullet is very short, and opens into the capa- 

 cious stomach. The latter consists of a large cardiac chamber 

 and a smaller pyloric chamber. It is lined by a transparent 

 chitinous cuticle, thickened along definite lines to provide 

 sclerites for the attachment of muscles and the grinding of the 

 food. It is also produced into setae, which act as filters. On 

 the roof of the stomach a combination of the sclerites produces 

 the gastric mill. The ossicles entering into its formation form 

 virtually a hexagonal framework provided with teeth. The 

 cardiac ossicle is a transverse bar on the top of the cardiac 

 chamber. The roof of the pyloric chamber is similarly pro- 

 vided with a shorter compact sclerite, the pyloric ossicle. 

 These two ossicles are joined by two short median pieces ; 

 the anterior of these is the precentral ossicle or urocardiac, 



Anus 



