66 BALANlDiE. 



surface of the terminal point of the rudimentary abdomen.* 

 The thorax is laterally compressed, the ventral surface 

 being very narrow, with the bases of the cirri placed closely 

 together. It consists, in appearance, of two very different 

 portions ; one a* soft, more or less rounded bag, which I 

 have called the prosoma ; and the other, which supports the 

 five posterior pairs of cirri, is narrower, invested with 

 stiffer membrane, and is more or less distinctly composed 

 of five segments. These segments (PI. 26, fig. 8) on their 

 dorsal and dorso-latcral surfaces, are generally driven like 

 wedges one into the other, with their points directed ante- 

 riorly : on the ventral surface the articulations are transverse. 

 The prolongation (e) of the thin membrane (a) surrounding 

 the anus (5), that is, the rudiment of the abdomen, which 

 sometimes carries caudal appendages, almost divides (in ap- 

 pearance, whether really I know not) the hindermost thoracic 

 segment along the medio-dorsal line, into two parts. I 

 have given the above drawing of these segments, but with 

 the dorsal surface much flattened, in Coronula diadema ; 

 in most species of Balanus, however, the wedges formed by 

 one segment being driven into another, are much sharper ; on 

 the other hand, in Xenobalanus they are nearly straight 

 and transverse. The three posterior segments are always 

 the most distinct ; the two next segments are also distinct 

 laterally, but along the dorsal surface they become, except- 

 ing in Xenobalanus and some few other cases, completely 

 confluent. The greater distinctness of the posterior segments 

 is conformable to what takes place in the higher Crustacea. 

 The articulations between the segments are folded in- 

 wards, and are formed of thin membrane, which in some 

 cases, as in Coronula diadem a ^ forms a marked contrast 

 with the much thicker, stiffer, and yellowish membrane of 

 the segments themselves \ in Balanus tintinnabulum, how- 



* Von Siebold and Stannius, in their 'Anatomie Ccmparee,' torn, i, p. 473, 

 and p. 440, foot-note), consider the articulated probosciformed penis as an 

 elongated abdomen ; a view which, at the commencement of my examination, I 

 was tempted to admit ; but the position of the caudal appendages on the dorsal 

 basis of the penis, suffice, I think, to show that this view is not correct; for 

 these caudal appendages evidently correspond with those borne on the very 

 extremity of the abdomen in the pupa. Nor, indeed, docs the position of the 

 anus accord well with such a view. 



