216 THE VOYAGE OF H.M.S. CHALLENGED. 



always in two very different pairs ; the two opposite sagittal canals (shorter ventral and 

 longer dorsal) are simply curved in the median plane of the nectosac ; the two paired and 

 equal lateral canals (right and left) are much longer, and more or less curved in different 

 directions, usually with three or four roundish loops, the course of which is often 

 characteristic of the single species. The top of the subumbrella, where the four radial 

 canals arise from the peduncular canal, is dislocated more or less towards the ventral side. 

 The points where the four radial canals open into the circular ring- canal, above the 

 insertion of the small velum, are sometimes marked by four red pigment-spots ; at other 

 times only two or three ocelli are preserved (an odd dorsal and two paired lateral) ; the 

 fourth (ventral) has disappeared (PI. XVIIT. figs. 8, 9). 



Siphosome. — The external form and internal structure of the siphosome, as well as 

 the character of the motions and habits, are so different in the various Agalmidee, 

 that we may divide this large family for that reason into two subfamilies : Crystallodinse 

 and Anthemodinse (25, pp. 39, 40). The siphosome of the Crystallodinaa (Agalma, 

 Crystallodes, &c.) is relatively short, about as long as or little longer than the nectosome ; 

 it is very little expansive and contractile, and distinguished by a peculiar rigidity, 

 caused by the peculiar shape and junction of the bracts ; these are very thick and 

 firm, prismatic or wedge-shaped, cartilaginous portions, so densely packed one over the 

 other that the intervals nearly disappear ; they form a complete carapace of scales 

 around the trunk even in its most expanded state. The motions of the siphosome in 

 the Crystallodinse, therefore, are A^ery weak and inconsiderable (compare PI. XVII.). 



The second form of the siphosome, very different from the first, is represented by 

 the Anthemodinse. It is very long and movable, usually much longer than the 

 nectosome, and in the expanded state often many times longer. The tubular trunk of 

 this siphosome is very extensible and contractile, and in the expanded state only 

 loosely covered by the bracts ; these are usually thin scales, often foliaceous, and 

 separated by dilatable intervals. The motions of the siphosome in these Anthemodina? 

 (Halistemma , Agalmojisis, &c.) are usually very active and quick ; the expanded stem 

 may be suddenly contracted and then occupy a much smaller space (one-tenth or less 

 of the expanded corm) (compare Pis. XIV.— XVI.). 



Cormidia. — The polymorphous persons and their organs are arranged and grouped 

 in the siphosome of the Agalmidse in a very variable order ; generally we may 

 distinguish ordinate and irregular cormidia ; there are, however, intermediate stages 

 between these two types. Perhaps the whole family may be divided according to this 

 difference into two subfamilies — Stephanomidse (with ordinate cormidia) and Halistem- 

 midaa (with irregular cormidia) ; in both subfamilies occur Crystallodinas (with rigid 

 siphosome) and Anthemodimie (with movable siphosome). The ordinate cormidia of 

 the Stephanomidse are equidistant and separated by internodes of equal length, which 

 are only covered with bracts. They exhibit a- most primitive and simple arrangement 



