MICROSCOPICAL STUDIES. 43 



fold of the integument grows around the trunk, and in the substance 

 of this mantle or pallial fold, five protective calcareous plates are 

 formed, one unpaired and the others paired. Early adult form is 

 now attained. 



When fully grown, and sexually mature, Lepas has the external- 

 appearance shown in Fig. 8, PL 5. The anterior end of the head 

 has become a great wrinkled peduncle or stalk, terminated posteriorly 

 in a shelly pouch, the mantle, enveloping the main mass of the body 

 and open only along a slit on the ventral border. Of the calcareous 

 plates strengthening this mantle, the larger of the two pairs, situated 

 towards the insertion of the peduncle, are termed the scuta (s) ; the 

 other pair placed at the far end of the mantle sac, are the terga (t), 

 while the unpaired plate, the carina (c), is a long and narrow keel 

 and separates, on the dorsal aspect, the opposite plates of the terga 

 and the scuta. 



Removing one side of the mantle, with its scutum and its tergnm, 

 the actual body of the Barnacle is exposed— an obscurely segmented 

 fleshy mass, bearing conspicuous tendril-like limbs. What corresponds 

 with the typical Crustacean head lies before these latter, and is 

 divided into 3 regions ; the anterior lies without the mantle and is 

 the peduncle ; the median is short and narrow and being attached 

 to the mantle and connected along the inner surface with the peduncle, 

 forms an " isthmus " ; the posterior, the most important, is large and 

 fleshy, and bears the mouth and its organs. 



Succeeding the posterior division of the head lies the partly 

 segmented limb-bearing thorax, and behind this again is found a very 

 small and rudimentary truncated abdomen terminating in two tiny 

 pointed processes. Tiny though it be, the abdomen is of importance, 

 for upon the dorsal surface is the anus, and it as well gives origin 

 to an organ many times larger than itself; an organ long, stout, 

 cylindrical, annulated and setose, that functions as the male copulatory 

 organ, and may be termed the penis. In life this organ is bent down 

 between the thoracic limbs, as shown in Fig. 5 (p), and not as 

 in Figs. 6 and 7, where it is straightened for the purpose of 

 clearness in the drawing. 



Of the appendages of the head, the anterior antennae persist in 

 a very minute and attenuated condition, at the anterior end of the 

 peduncle, where they lie embedded in the attaching cement ; 

 posterior antennae are absent, and the mouth parts are much reduced, 

 forming a small eminence surrounding the mouth. The parts com- 

 prise an upper lip or labrum with labial palps, two mandibles and 

 four maxillae, of which the hinder pair form a lower lip. 



The thoracic appendages or feet, six pairs in number, are all 

 tendril-like, long, slender, many-jointed, and closely set with a double 



