208 THE VOYAGE OF H.M.S. CHALLENGER. 



long tentacle (t), bears a hemispherical basigaster with a thick exodermal wall full 

 of cnidocysts (sb). The large ovate or fusiform stomach (s) possesses four or six 

 strong hepatic ridges (sh), composed of small villi and coloured red or brown (four in 

 Dieymba and Apolemopsis, six in Apolemia). The long tubular proboscis (sr) bears a 

 corresponding number (four or six) of longitudinal muscle-bands, and at the distal mouth 

 a similar number of small rounded lobes. 



Tentacles and Palpacles.- — All Apolemidse bear very numerous long and thin fila- 

 ments, which are usually described as tentacles. According, however, to the general 

 definition, which holds good for other Siphonophorse, we may call only those larger 

 filaments which are attached (singly) to the base of the siphons tentacles, whilst we 

 call the similar smaller filaments which arise from the base of the cystons and palpons 

 palpacles (thus a single one from each polypite). The structure is otherwise the same 

 in both. They are simple, thin and long cylindrical tubules, closed at the distal end, 

 opening into the pedicle of the polypite at the proximal end ; they never bear lateral 

 branches. The structure of the cylindrical wall is the usual one, a vibratile entodermal 

 epithelium inside, a cnidal exodermal epithelium outside, and between them an elastic 

 fulcrum, which bears inside a thin plate of ring-muscles, and outside numerous radial 

 ribs covered with longitudinal muscles. The cnidocysts are larger and more numerous 

 in the abaxial part of the exoderm and towards the distal end, often arranged in two or 

 four longitudinal series. 



Cystons. — Each siphon or feeding polypite of the Apolemidse is associated with a cyston 

 or excreting polypite (fig. 2, y). The Dicymbidas, therefore, have in each monogastric 

 cormidium a single cyston, the Apolemopsidse in each polygastric cormidium several 

 cystons, two to four or more. The cystons or " anal vesicles " are intermediate in size 

 between the larger siphons and the smaller palpons, and at once distinguished from both 

 by the dark (red or brown) colour of their nontransparent body. Their four segments 

 are usually not so distinct as in the similar but more fully developed siphons ; the 

 hepatic ridges of the latter are replaced by series of glandular villi, secreting a granular 

 red or brown pigment-mass. This is accumulated in a vesicular terminal diverticulum 

 of the proboscis, and may be ejected through a small opening of the latter (anus). 



Palpons. — The Apolemidse possess exceedingly numerous tasters or palpons, usually 

 twenty to forty or more in each single cormidium, therefore several thousands, when 

 the number of cormidia exceeds a hundred, as is often the case in corms of two or 

 three metres in length. The palpons are slender, very mobile, cylindrical or spindle-shaped 

 tubes, closed at the pointed distal end, which is richly armed with cnidocysts and long 

 cnidocils. The middle part of the utricle exhibits sometimes four or six longitudinal 

 ridges, corresponding to the hepatic stripes of the siphons and the excretory pigment- 

 stripes of the cystons. The pedicle of each palpon bears a long, very thin palpacle, 

 similar to that of the cyston (compare 7, pi. xviii. fig. 4, and 8, Taf. xii. p. 10). 



