342 COMPARATIVE AX ATOMY CHAP. 



opposite walls, mingle in such a way as to form a fine sieve apparatus, only 

 admitting of the passage of fluids. The typhlosolis, which is fastened to the body 

 wall by a pair of strong muscles, is followed by a third extremely muscular division, 

 which is called Rathke's organ, and carries on energetic rhythmic contractions. 

 A small tubular blindly closed terminal division, which is the only representative 

 of the mid-gut, receives the ducts of two large contractile hepatic tubes. A hind- 

 gut and anus are wanting. The whole intestine stretches only to the 3d thoracic 

 segment. Portunion mcenadis sucks the blood of its host (Carcinus). The peculiar 

 structure of the enteric canal, in which Rathke's organ and the cephalogaster alter- 

 nately contract and expand, seems adapted to this sucking process. 



The enteric canal of the minute male of Portunion which lives in the body of the 

 female does not show the peculiar transformation which it undergoes in the female. 

 It is straight, and possesses two hepatic tubes, a hind-gut, and an anus. 



V. The Nervous System. 



The nervous system of the Crustacea is constructed on -the same 

 type as that of the Annulata, and must be derived from the latter. 

 The result of research in comparative anatomy and ontogeny justify 

 us in giving the following diagrammatic representation of its general 

 structure and original constitution. This scheme stands in direct 

 relation to the generalised plan of the segmentation of the body of 

 the Crustacean sketched above (p. 300). In the most anterior segment 

 of the body (head segment) the brain (supra-cesophageal ganglion), 

 consisting of two symmetrical lateral halves, lies in front of and over 

 the oesophagus, giving off nerves to the unpaired eye, the anterior 

 antennas and the frontal sensory organs (see below). Each of the 

 other segments of the body possesses two ganglia (a double ganglion) 

 lying very near each other in the ventral middle line. The two 

 ganglia of each segment (the two symmetrical halves of each double 

 ganglion) are connected together by means of a transverse commissure, 

 and with the corresponding ganglia of the preceding and subsequent 

 segments by longitudinal commissures. The two most anterior 

 longitudinal commissures which connect the double ganglion of the 

 second segment with the brain embrace the oesophagus. These are 

 the cesophageal commissures. The whole central nervous system thus 

 consists, as in the Anmilata, of the brain and the segmented ventral 

 chord (ventral ganglionie chain), whose segmentation corresponds 

 with the segmentation of the body. From each double ganglion of 

 the ventral chord nerves proceed to the body musculature of the 

 segment to which it belongs, and to the musculature of the limbs 

 with which that segment is provided. There are therefore one 

 double ganglion for the 2d pair of antennae (in the 2d segment), a 

 similar one for the 2 mandibles (in the 3d segment), 2 pairs of 

 ganglia for the anterior and posterior pair of maxillae (in the 4th and 

 5th segments), and so on, a pair of ganglia in each segment for 

 the limbs Avhich belong to it. 



It must, however, be specially noted that the assumption of a special ganglion for 



