56 THE COCKROACH : 



appendages, though it is unlikely that such is their real 

 character. No means at present exist for identifying the terga 

 and sterna of the head, nor have the gena, the occipital frame, 

 and the cervical sclerites (described below) been assigned to 

 their segments.* It is worthy of notice that in the stalk-eyed 

 Crustacea, the head, or what corresponds to the head of Insecta, 

 consists of either five or six somites, taking into account a 

 diversity of opinion with respect to the eyestalks, while only 

 four pairs of appendages can be certainly traced in the head of 

 the Insect. The mandibles and maxilla) exist to the same 

 number in both groups, and are homologous organs, so far as is 

 known ; the numerical difference relates therefore to the antennae, 

 of which the Crustacean possesses two pairs, the Insect only 

 one. Whether the pair deficient in the Insect is altogether 

 undeveloped, or represented by the pair of prominences which 

 give rise to the labrum,f is a question of much theoretical 

 interest and of not a little difficulty. 



mj 



The following table shows the appendages of the head and 

 thorax in the two classes. The homologies indicated are, how- 

 ever, by no means established. J 



CRAYFISH. COCKROACH. 



Antennae. 



Eyestalks. 



Antennules. 

 Antennae. 



Mandibles. 

 Maxilla? (1). 

 Maxillae (2). 



Maxillipeds (1). 



Maxillipeds (2). 

 Maxillipeds (3). 



Mandibles. 

 Maxilla* (1). 

 Maxillae (2). 



Thoracic Legs (1). 

 Thoracic Legs (2). 

 Thoracic Legs (3). 



* "I think it is probable that these cervical sclerites represent the hinclermost of 

 the cephalic somites, while the band with which the maxilla? are united, and the 

 genre, are all that is left of the sides and roof of the first maxillary and the mandi- 

 bular somites. "--Huxley, Anat. Invert. Animals, p. 403. 



t Balfour, Embryology, Vol. I., note to p. 337. 



I J. S. Kingsley in Q. J. Micr. Sci. (1885), has reviewed the homology of Insect, 

 Arachnid, and Crustacean appendages, and comes to conclusions very different from 



