DESCRIPTION OF PLATE, 25. 663 



t, tergum. 



a, adductor scutorum muscle, with the scutum on the near side 



removed. 



b, the whole space enclosed by a broken sinuous line, round a 



and b, shows the cut surface of attachment to the near 

 scutum, which has been removed. 

 b', lower muscle, on the near side, running from near the basal 

 edge of the scutum to near the basal margin of the labrum 

 (e). Above this are three other muscles (all on the near 

 side), running to the membrane between the labrum and 

 adductor muscle. 



c, prosoma, including the main portion of the stomach and 



thickened ends of the vesiculse seminales: homologically this 

 is formed by the development of the second thoracic seg- 

 ment, which carries the first pair of cirri ; and possibly, in 

 part, by the antecedent segment, i. e. the first thoracic 

 segment. 



c', thorax, extending from the prosoma to the posterior end of 

 the body : the letter (c) stands on the segment supporting 

 the third cirrus ; homologically, this segment is the fourth 

 thoracic. 



d', orifice of the acoustic sack, above which is the basal articu- 

 lation of the first cirrus. 



e, labrum, forming the back (i. e. anterior end) of the mouth, 

 with the transverse palpi obscurely seen on the summit. 



/, sack in which the animal lies, with the tunic lining it, con- 

 tinuous with that investing the prosoma (c), and lining 

 the under sides of both opercular valves, but cut off round 

 (b) and (a) by the removal of the near scutum. The 

 branchia on the further side, which occupies the position 

 represented at fig. 3, is covered by the body of the animal. 



g g, ovarian, inosculating cseca, branching from the simple 

 duct (of which the near one of the pair is represented), 

 leading to the ovaria (not represented) within the body. 



h, rostral depressor muscle of the scutum : the lateral depressor 

 muscle of the scutum is hidden by the body and by the 

 membranes of the sack. 



i, carinal depressor muscle of the tergum. 



z, antennae (three terminal segments) of the pupa ; I distinctly 

 saw these in this species, but they are here represented 

 considerably too large, even on the supposition that a 

 young shell had been drawn, and magnified about twelve 

 times. 

 Fig. 2, Testes o£ B alarms perf or atus, greatly magnified. 



3, Branchia of Bal. tintinrtabulum, on the same scale as in fig. 1, 

 and in its natural position. This drawing was made by 

 lifting up the body in fig. 1 ; the organ being thus com- 

 pletely exposed over its whole interior surface ; a is the 

 basal end of the spur of the tergum. 



