10 



Each gUl consists of a multitude of stiffisli vascular filaments, arranged 

 round a central stem, to form an elongate-pyramidal plume or brush. Very 

 long thread-like setx are also connected with the gill-plumes, and are numerous 

 enouo-h to form by their interlacing a sort of adjustable straining apparatus to 

 keep the gills from getting clogged by dirt. 



The structure and arrangement of the gills are of the highest value in the classification of 

 the higher Crustacea. On this subject see Huxley, on the Classification and Distribution of the 

 Crayfishes, Proceedings of the Zoological Society for 1878, pp. 752-788. 



7. Of the Digestive Si/stem. 



The dio-estive system of Nephrops consists of the alimentary canal and of a 

 pair of large digestive glands. 



The alimentary canal is a tube, of which the anterior part is greatly ex- 

 panded to form a stomach : it is for the most part thin-walled, and it passes w^th a 

 single curve, but without any coiling, straight through the body from the mouth, 

 which lies behind the 4th (mandibular) sternum, to the anus, which perforates 

 the ventral surface of the telson. 



The mouth is a longitudinal median chink, bounded in front by an over- 

 lapping fold of the integument termed the labrum or upper lip, and behind by a 

 pair of fleshy leaves known as the metastoma or lower lip. On either side of the 

 mouth are the powerful mandibles, upon which, like the leaves of a book, lie the 

 two pairs of maxillae and the 1st pair of maxillipeds, these being closely followed 

 by the last two pairs of maxiUipeds. 



A short " gullet " leads upwards from the mouth into the capacious 

 stomach : like the stomach, it is lined by an invagination of the cuticular exoskele- 

 ton which is cast off every time the animal moults. 



The stomach is divided into two chambers — a wide anterior one in the roof of 

 which is a powerful crushing-apparatus formed by three gastric teeth, one median 

 and two lateral, and a narrower posterior chamber furnished with hairs which 

 interlace to form a filter. 



From the posterior or jiyloric chamber the narrow intestine passes straight 

 back to the vent. 



On either side of the stomach and anterior part of the intestine, the cavity 

 of the body is filled by a large digestive gland. This paired gland consists of a 

 vast number of tubules, which at length discharge into a duct that opens into 

 the anterior part of the intestine on either side. 



8. Of the Circulation of the Blood. 



The blood is colourless, and the circulatory system consists of a heart and 

 thin-walled blood vessels. 



