158 THE INVERTEBRATA 



numerous individuals are budded. Most of these are polyps (hy- 

 dranths), distinguished from those of the Calyptoblastea by the fact 

 that the perisarc stops short at the base of the polyp and does not 

 form a hydrotheca. The medusoid individuals take their origin directly 

 from the coenosarc each as a simple bud, within which is developed 

 a single medusa which eventually divests itself of a thin covering, 

 breaks from its stalk and swims away. Several may spring from the 

 same stem, but this may also bear normal polyps. There is here no 

 blastostyle, or polyp modified for budding off medusae, and this con- 

 dition, in which polyps and medusae belong to the same grade of 

 differentiation from the coenosarc, is possibly to be regarded as 

 primitive, that of Obelia as secondary. In Eudendrium an intermediate 

 stage occurs. Medusae are budded off from the stalk of a normal 

 polyp, and as soon as this budding commences the polyp loses its 

 tentacles, diminishes in length and may be said to become a blasto- 

 style. 



Tiibularia (Fig. 117) occurs as a colony of large polyps with long 

 stalks springing from a hydrorhiza of insignificant extent. At the base 

 of the polyp the stalk forms a swelling ; there the perisarc stops. There 

 is an oral cone surrounded by a ring of tentacles and also a ring of 

 larger (aboral) tentacles at the broadest part of the polyp. Both kinds 

 of tentacles are solid, with an axis of vacuolated endoderm cells 

 placed end to end, which have a skeletal value. In Fig. 118 part of 

 the phenomenon of digestion is illustrated. A crustacean has been 

 swallowed and lies in the stomach {s). After preliminary digestion a 

 fluid mass of half-digested material is formed and, by alternate 

 contraction of A the stomach and B the spadix (manubrium) of the 

 gonophore together with the basal swelling of the polyp, the food is 

 forced into contact with all the absorptive epithelium of the polyp 

 and gonophore and also pipetted along the cavity of the stalk. 



The reproductive individuals originate from hollow branched 

 structures springing from the polyp itself between the oral and aboral 

 tentacles. Each polyp has several of these branches, and from each 

 branch a number of reproductive individuals arise. The branch is 

 usually termed a blastostyle, although it is only part of an individual 

 and not a modified polyp as in Obelia. Each of the buds it produces, 

 however, has the structure of a medusa but remains attached to the 

 parent polyp as long as it lives. Like the free-swimming medusa of 

 Bougainvillea it conforms to the anthomedusan type, the depth of the 

 medusa bell exceeding the width and the gonads being situated on the 

 manubrium (spadix). This sessile medusa is called a gonophore. As 

 seen in Fig. 119 A, the radial and circular canals are formed as in 

 Obelia, and four very short tentacles occur opposite the radial canals 

 on the margin of the bell ; but the entrance to the subumbrellar cavity 



