10 
THE VOYAGE OF H.M.S. CHALLENGER. 
b. Anatomy of the Male of Scalpellum regium. 
I will now proceed to give an anatomical description of the complemental male of 
Scalpellum regium (Wyv. Thoms.), Hoek. I choose this species because it is represented 
by numerous specimens, and also because it is one of the largest species in the Challenger 
Collection. 
Form and dimensions. — The complemental male of Scalpellum regium has an 
elongated oval shape. Its length varies from 1'6 to 2 '4 mm., its breadth is 0'63 to 
0'71 mm. The difference in length corresponds to differences in some of the internal 
parts, especially of the testis. Whether it is occasioned by the growth of this organ I 
cannot say. The third dimension, the thickness, is nearly equal to the breadth. We 
may call the extremities of the longer axis the poles of the body, and I propose to call 
one the peduncular, the other the capitular pole (PI. I. fig. 1). 
The Antennae. — The only appendages visible externally are the small antennae, 
situated close to the extremity of the body corresponding to the peduncle of other 
Cirripedia ; they are seated at a little distance from the extremity, on that side of the 
body which represents the ventral surface. They have two segments ; one cylindrical, 
and about twice as long as the other, which is flat and triangular. What Darwin calls 
the third and ultimate segment of the antennae is very distinct in the case of this little 
creature (PI. I. fig. 3). It is articulated to the upper surface of the disk, and directed 
rectangularly outwards. Whereas the main segments of the antennae are not furnished 
with spines, this latter segment bears five spines at the end, and three very slender ones 
at a notch a little beneath the extremity of this segment. 
With the aid of these antennae the little creatures are attached to the inner surface 
of the scutum of the hermaphrodite or female. The triangular terminal segment of the 
antennae, in all the cases I observed, surrounded the extremity of a transparent mass, which 
I think can safely be considered as the product of the cement-glands which are in relation 
with the antennae. It is by means of this cement that the attachment of the triangular 
disk takes place. In the case of Scalpellum regium the males are attached a little above 
the adductor muscle, and, as a rule, three of them are implanted so closely together as to 
touch each other. What I think very peculiar is, that in three different cases observed 
by me, two of the three males attached to the scutum were much further developed than 
the third ; the first contained a fully developed testis and a well-filled vesicula seminalis, 
the third was still in the condition of a Cypris-larva (PI. II. fig. 3), probably only lately 
attached to undergo its final metamorphosis. 
The 'wall of the body is a chitinous skin, which is comparatively thin and delicate ; 
when a transverse section of the body is made, the chitinogenous epithelium beneath the 
