of the Fishery Board for Scotland. 



103 



apart, throwing the second penis outside, but at the same time a longitudinal 

 septum (ra., figs. 37, 59, 60) has appeared which continues the tube. It is 

 merely a continuation of the side of the penis by a soft flexible membrane 

 instead of by the hard chitin wall. The second penis is situated behind 

 the first, and when it is introduced into the first penis it crosses over this 

 membrane, which yields readily to pressure. In length the membrane is 

 short ; it is united below to the basal bone and forms the tissue binding 

 that bone on one side to the tubular part of the first penis. In fig. 10 is 

 shown a transverse section through the base. The basal bone (b., fig. 59), 

 has a large segment cut out of it, leaving its proximal part simply a 

 narrow rim to which the membranous septum is attached. The membrane 

 stops just a little beyond the point where the inturned edges of the penis 

 meet and form the tube. 



The tube of the penis opens in the base on the anterior side. The 

 posterior side of the beginning of the tube is formed by the membrane. 

 The genital papilla is inserted in the beginning of the tube. When 

 the second penis is in the first, its broadened base lies on the posterior 

 surface of the basal joint. Any pressure of the second penis due 

 to its movement is transmitted through the membrane to the genital 

 papilla (fig. 60). Moreover, as will be shown later, the second penis moves 

 up and down in the first in a manner similar to that of the plunger of a pump; 

 so that sperms or spermatophores ejected from the vas deferens into the penis 

 tube will be pumped up and out of it. The groups of hairs that are found 

 on the wing of the chevron and round the basal joint act as valves or 

 packing round and in the beginning of the tube. 



The Second Penis. 



The second penis is rod-like. It consists of three main parts, first an 

 arm from the end of which the rod rises at right angles (ar., figs. 65, 61, 

 etc.). This arm, which is fused to the ventral edge of the second joint, is 

 formed in its lower half of chitin and in its upper part of soft membrane, 

 in figs. 65 and 68. The arm is the immovable part of the second penis. 

 From its posterior extremity rises the movable penis. It consists of two 

 parts, viz., a base and the rod. The base consists of twobones, a. and b., 

 figs. 53, 54, and 56, loosely connected together and to the proximal end of 

 the rod with soft membrane. The largest bone is of a tooth-shape. It 

 consists of a rather broad tooth rising from an expanded base. The other 

 is a narrow somewhat bow-shaped bone. The loose integument between it 

 and the other basal bone permits of the former folding over towards the 

 latter to a considerable degree. 



The proximal end of the rod is expanded and cut obliquely off (ib.). 

 Distally the rod tapers, at first rapidly then gradually, up to about two- 

 thirds of its length, where there is a joint permitting a slight amount of 

 movement. The loose part of the rod is curved, with the convexity forward. 

 At the joint there is on the anterior side a little tuft of long spine-like 

 teeth (fig. 104). Above the joint the rod tapers more, and it is curved 

 in the opposite sense to the proximal portion. The tip bears a depressed 

 oval cap set obliquely on the end ; it is fringed with teeth (fig. 105). The 

 top of the rod is cast slightly in towards the median line. 



The Muscular System. 



The Abdomen. — The posterior edge of the carapace has attached to its 

 under surface on each side a membranous plate directed forward into which 

 a muscle is inserted. This plate is attached by a strong membrane to the 

 edge of the outer chevron, and the muscle is inserted into the posterior 



