458 COMPARATIVE ANATOMY CHAP. 



II. The Integument and Glands. 



The integument is of the same type as that of the Crustacea and 

 all other Arthropoda, the body being covered by a chitinous cuticle 

 forming an exoskeleton. This cuticle is of varying flexibility and 

 thickness, and shows many and great modifications, not only in the 

 various parts of the body and limbs of the same animal, but among the 

 different members of the class. A knowledge of the constitution of 

 the exoskeleton in the various parts of the body is of great importance 

 in classification, as is also a knowledge of the setee, hairs, scales, etc., 

 which belong to the category of cuticular formations. The epithelium 

 which secretes the chitinous cuticle is here also called the hypodermis. 



During eedysis, which accompanies the metamorphoses of the 

 Antennata and the growth of the larva?, the whole exoskeleton is 

 thrown off, together with the chitinous intima of that part of the 

 intestine thus lined, the chitinous intima of the tracheae, and the ducts 

 of the glands. The chitinous integuments thrown off are known as 

 exuvia. 



Dermal glands are widely spread in the Antennata ; they appear 

 in a great number of modified forms, emerge at the most various points 

 of the body, and form secretions differing greatly in constitution. A 

 comparative study of these on a wide basis is urgently needed. 

 Investigation is especially needed as to which glands in the Antennata 

 correspond with the coxal and spinning glands of Peiipatus, and which 

 glands, if any, are to be considered as transformed nephridia. At 

 present the observations in comparative anatomy and ontogeny neces- 

 sary to enable us to give a definite answer to these questions are 

 wanting. 



Among the glands emerging on the outer integument we may first mention the 

 salivary glands, which open in the immediate neighbourhood of the mouth. They 

 are everywhere found in the Antennata, generally lying at the side of the fore-gut, 

 in the head or thorax, and occur in 1-3 pairs. They are either simple or much lobed 

 acinose glands, or else fall on each side into 2 or more glandular sacs. The ducts of 

 the glands of each side, however, nearly always unite in a common duct, which 

 finally unites with the duct from the other side to form an unpaired canal, which 

 usually emerges externally on the lower lip or the hypopharynx, but in any case in 

 the immediate neighbourhood of the mouth. Not infrequently a vesicular appendage 

 (saliva reservoir) is found on each side of the canal. It sometimes happens that 

 there are two separate apertures for a single pair of salivary glands, the unpaired 

 terminal portion being absent. Where there are several pairs of glands, their ducts 

 may also emerge separately ; usually, however, the ducts from each pair imite to form 

 a common terminal portion. 



The salivary glands are, as far as their development is known, invaginations of 

 the oral edge of the stomodseum. According to some observations, it appears that 

 the unpaired duct forms secondarily, the 2 salivary glands proceeding from paired 

 rudiments. 



Spinning glands (sericteries) occur in many Insect larra, and are specially strongly 

 developed in those which pass through a pupa stage (e.g. caterpillars of Lcpidoptt ra 



