130 



The Naturalist. 



annulose, consisting of eight segments. The first, or cephalic, 

 segment, is the largest and most important, lodging the masticatory- 

 organs, gullet, stomach, liver, brain and eye. It is homologous with 

 the first seven somites of a typical crustacean fused into one ; the 

 bivalved carapace being an enormous expansion of the three 

 mandibular somites. This development of the cephalic somites is, 

 we shall find, accompanied by a consolidation of its nervous system, 

 in harmony with the law governing all Crustacea, in which a separa- 

 tion of the nervous cord into distinct ganglia is coincident cwith a 

 corresponding decrease in the importance of the carapace. Suspended 

 from the second (thoracic) and the four succeeding segments are five 

 pairs of jointed feet, the middle three of which carry branchial plates, 

 furnished with long plumose set^e or bristles. Along the ventral 

 aspect of the same segment, runs a depression or gutter into which 

 the particles of matter, drawn within the shell by the incessant move- 

 ments of the feet, are directed by the first pair, the rejected material 

 being extruded by the last pair. The masticatory organs consist of 

 a pair of toothed jaws, and a labrum. The latter lies horizontally paral- 

 lel with the mouth, to which, part of its base is attached. It works 

 vertically, between the two rows of feet. Its free extremity is expanded 

 into a fleshy contractile palpiform process studded with short hairs. 

 When in use it extends beyond the ventral edges of the carapace or 

 shell. The first and second maxillae are fused with the long narrow 

 mandible, terminating in a surface not unlike that of an elephant's 

 tooth, and are suspended vertically from before the first articulation, 

 having a lateral semi-rotatory movement. 



The food particles drawn forward by the labrum are seized by the 

 jaws and passed to the mouth, which lies immediately in front thereof, 

 and leads into a short narrow sesophagus that curves forward and 

 upward, opening into the stomach just behind the brain. While the 

 food is undergoing the process of digestion it is. slowly passed back- 

 wards and forwards by the alternate dilatation and contraction of the 

 muscular walls of the stomach. From each side, near the anterior 

 orifice of the stomach arises a coecal appendage, in shape like an S 

 inverted. The free end is closed, the other occasionally pours a 

 colourless fluid into the stomach when food is lodged therein. When 

 in use a transverse contraction, synchronous with the pulsation of the 

 heart, is visible near the bend of the organ ; and at times portions of 

 food may be seen to have a swaying movement within, similar to what 

 is observable in the stomach. But these movements in the cascals 



