REPORT ON THE SIPHONOPHOR.E. 191 



concave lower (or ventral) .side ; the proximal end being attached to the trunk by a short 

 pedicle, which can be raised and lowered by a muscle. The peripheral margin is usually 

 thin, often with a few teeth. Sometimes the dorsal face is provided with a prominent 

 median crest, or a number of parallel or divergent ribs (three to five or more) which are 

 armed with cnidocysts and prominent at the distal end as free teeth. More rarely the 

 bracts are very thick and compact, either roundish clubs (in the Apolemidse) or prismatic 

 bodies (in the Crystallodinse). In these latter they are so thick and densely apposed one 

 to another, that the movable stem loses its contractility and the siphosome becomes rigid. 

 Each bract contains a simple canal, which arises from the trunk, runs along the ventral 

 side of the bracts (usually in the median line), and ends blindly near to its distal end. 



Siphons. — The Physonectaj have usually large and well-developed siphons, in which 

 the four usual portions or segments may be distinguished ; these exhibit, however, 

 a rather various development in the different groups. The pedicle, or the first portion, 

 is usually a short cylindrical tubule ; but it is longer in some Agalniidse, and very 

 prolonged (similar to a very long lateral branch of the trunk) in the Forskalidaa. The 

 basigaster, the second segment, is usually small, hemispherical or ovate, with a narrow 

 cavity and a thickened exodermal wall, full of cuidocysts ; it is often elongated and 

 pyriform, in the Brachystelia (mainly in the Discolabidaa). The true stomach, or third 

 portion of the siphon, is usually the largest part, with a wide and very extensible 

 cavity, the exoderm of which is very muscular, the entoderm glandular. The hepatic 

 glands are usually developed in the form of long parallel hepatic ridges, more rarely in 

 the form of numerous scattered hepatic villi, as in the Athoridse and Brachystelia 

 (Nectalida?, Discolabidaa, Anthophysidse), also in the gigantic Bathtjphysa among the 

 Forskalidaa. There is, however, no sharp boundary between these two forms of 

 liver ; sometimes the hepatic villi are arranged in regular longitudinal series and thus 

 pass over into true liver-ridges (" Leberstreifen "). The number of the latter is usually 

 eight, more rarely four, six, twelve, or sixteen ; sometimes four larger perradial ridges 

 alternate regularly with four .smaller interradial, and between these are interpolated 

 eight shorter adradial ridges (PI. IX. fig. 7). The proboscis, or fourth and last portion of 

 the siphon, is usually a cylindrical, very mobile and contractile tube ; its distal mouth 

 opening may be expanded in the form of a very large and thin suctorial disc, sometimes 

 circular, at other times polygonal, often octagonal or square. The edge of the mouth 

 is usually armed with peculiar cnidoblasts and palpoblasts. The outside of the siphon 

 is often covered with vibratile epithelium, especially the proboscis. 



Tentacles. — Each siphon bears a single tentacle attached to its basal portion, either to 

 the distal part of the pedicle, or to the basigaster itself, often in a constriction between 

 them. The tentacles in all Pkysonectas are very kmg and contractile, cylindrical, 

 tubular filaments, of the same structure as the trunk, with an outer strong layer of 

 exodermal longitudinal muscles, and an inner thin layer of entodermal circular muscles. 



