510 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXVIII. 



B. INTERNAL MORPHOLOGY. 



Body wall. — The bod} T wall is made up of three layers (1), the out- 

 side layer of chitin cells forming the exoskeleton or cuticle (c. fig - . 16); 

 (2), the cellular "hypodermis" (h); (3), the connective tissue lamina 

 which lines the entire body wall and supports the alimentary canal and 



Fig. 16.— Section of body wall of Lepeophtheirus pectobalis. (After Claus.) c, cuticle or 

 chitin exoskeleton; at., connective tissue endoderm; h, cellular hypodermis. 



other organs (c. t.). There is no continuous body cavity, but only a 

 series of lacunae, through which the colorless blood is driven in spas- 

 modic currents. Furthermore these lacunae are so situated that no 

 extensive circulation is anywhere possible, but there is instead a series 

 of limited circuits which interlace with one another. 



ALIMENTARY CANAL. 



The mouth opens directly into a short, curved oesophagus, which 

 passes backward between the supra and infracesophageal ganglion, 

 along the dorsal surface of the latter, and enters the ventral surface of 

 the stomach a little behind its anterior end. It is very narrow through- 

 out its entire length and is easily overlooked in both longitudinal and 

 transverse sections (oe. fig. 17). The stomach is many times the 

 width of the oesophagus, but is only a little wider than the intestine. 

 It is situated close to the ventral surface of the carapace and stretches 

 from just behind the eyes backward along the mid line at least to the 

 center of the thoracic area (st.). 



It is somewhat wider in front than posteriorly, and is also raised 

 away from the ventral surface above the posterior end of the infra- 

 oesophageal ganglion. This raised end is produced into a median lobe 

 or ceecum, reaching forward over the oesophagus and between the 

 ovaries or testes. The stomach tapers uniformly toward the posterior 

 end, where it opens directly into the intestine (i). There is a con- 

 striction at this point like the beginning of a sphincter muscle, but the 

 opening can not be closed. The intestine, therefore, is a direct continu- 

 ation of the stomach; it enlarges considerably just back of the stomach, 

 then contracts in the fourth thoracic segment, enlarges again in the 



