28 bulletin: museum of comparative zoology. 



ticula, the margins of which approach each other without uniting, as is 

 shown by transverse sections (Figs. 26 and 27). This linea ventralis 

 may be traced hack along the median ventral line of the body as far as 

 the lobes of the ventral tube, which is a median appendage of the first 

 abdominal segment ; between the body segments, however, the liuea 

 ventralis suffers considerable interruption. I shall return to this 

 peculiar structure when considering the glands of the head. 



The morphology of the labium of Collembola has never been eluci- 

 dated, and is difficult to understand, even after careful comparison with 

 other orders of insects. I am at present able to offer only a few sugges- 

 tions upon the subject. The glossa and paraglossse have no connection 

 with the labium, although fused with each other. Labial palpi have 

 been regarded as absent by other authors, yet the anterior regions of 

 the labium, which are doubtless tactile in function, may perhaps be 

 palpi in a morphological sense also. If a maxillary palpus of this same 

 insect with its setigerous tubercles be imagined as having become sessile, 

 through a shortening of the stalk, we have a counterpart of the terminal 

 lobe of the labium. I see no serious objection to considering the 

 two remaining sclerites of either side as mentum and submentum, 

 although the presence of a nearly obsolete suture, tending to divide 

 each submentum into two sclerites, certainly complicates the matter. 



The movements of the labium are effected by five pairs of muscles, of 

 which three are elevators and two depressors. I shall describe these 

 muscles as they appear in successive sagittal sections, beginning at the 

 median plane. 



1. Posterior depressor. This is the shortest of the labial muscles, and 

 is inserted on the ventral wall close to the median plane (Plate 1, Fig. 3, 

 and Plate 3, Fig. 25, 1. dep.p.). This muscle arises from the dorsal sur- 

 face of the thick chitinous wall of the salivary duct and runs forward, 

 downward, and inward to its insertion. It opens the mouth by pulling 

 upon the under surface of the lower lip. All the lateral muscles are seen 

 tranversely sectioned in Plate 4, Figures 28-31. 



2. Mesal elevator. This muscle runs along the upper and inner margin 

 of each half of the labium (Figs. 3 and 25, 2. Ivt.ms.). The insertion is 

 at the anterior extremity, and the origin close beside that of the preced- 

 ing muscle, on the same chitinous duct. The function is to close the 

 mouth by diminishing the extent of the inner surface of the labium. 



3. Anterior depressor (Fig. 3, 3. dep. a.). In origin, course, and 

 function this muscle is similar to the posterior depressor; it is more 

 lateral, however, and is inserted somewhat in advance of its companion. 



