30 COMPARATIVE ANATOMY. 



two elements, the distal one of which is grooved on its inner 

 face, and is capable, by being applied to its fellow, of forming a 

 short cylindrical tube which probably conveys the male semen 

 to the vulva of the female. 



The ' cephalo-thorax ' is formed by the fusion of the terga of 

 the cephalic, the thoracic, and the abdominal segments. The 

 cephalo-thoracic pleura are well developed, and extend so far 

 downwards as to hide the bases of the appendages. 



The sternal portions of the pleura are membranous and are 

 reflected upwards to the roof of the cephalo-thorax, and then, 

 after being deflected, become continuous with the partially 

 calcified ' epimera.' In this specimen this membranous por- 

 tion has been torn through by the separation of the tergal half 

 of the cephalo-thorax from the coalescent sterna, which form the 

 ventral surface of this part of the animal. 



On the dorsal surface the ' cephalo-thorax ' does not present 

 any indications of division into distinct terga, corresponding to 

 the sterna and appendages of the ventral surface, but neverthe- 

 less contains areas more or less distinctly marked out. A groove 

 commences at the base of the antennae, and, after passing 

 directly backwards for a short distance, curves upwards and 

 fuses with the corresponding curve of the opposite side at about 

 the middle of the dorsum. The part in front of this suture has 

 been called the ' cephalo-stegite,' and the portion behind the 

 ' homo-stegite.' The ' homo-stegite ' is further divided by two 

 lateral linear depressions, one on each side of the median line, 

 called the ' brancho-cardiac sutures,' and by a very faint trans- 

 verse ridge, which is to be seen a little way behind the 

 cervical suture, into a quadrangular area which corresponds 

 externally with the position of the heart internally. In this 

 specimen this area is tolerably well indicated by a boundary 

 line of irregularly shaped white spots. The cervical suture indi- 

 cates the line of separation between the cephalic and thoracic 

 segments. The area between the cervical suture in front, 

 and the faintly marked ridge behind, represents the confluent 

 terga of the three thoracic segments, while that part of the 

 'homo-stegite' posterior to this corresponds with the terga of the 

 five abdominal segments. The lateral areas which lie on the side 

 of these median divisions represent the conjoined pleura of the 



