SEA-FIR. 



247 



blastea, with fixed plant-like colonies and all parts save the hydranths protected by 

 a perisarc ; (3) Campanulariae or Calyptoblastea, with fixed plant-like colonies, the 

 hydranths lodged in chitinous thecae formed by the perisarc; and (4) Trachy- 

 medusae, a group of Medusae which have no fixed colonial forms, but develope as 

 Medusae from the egg. The Sea-fir belongs to the Campanulariae. 



In addition to the possession of hydrothecae, a Campanularian differs from a 

 Tubularian in three important respects : the terminal hydranth of a stem or branch 

 is the youngest and not the oldest in the stem or branch ; the generative zooid is 

 with few exceptions borne on a blastostyle ; and when that zooid is a Medusa it is 

 furnished, as a rule, with auditory sacs, and not with eye-specks. Hence the Medusae 

 of Campanularians are classed as Vesiculatae, those of Tubularians as Ocellatae. 



The Medusa-form in the order Hydroidea has the following general cha- 

 racters : 



A. 



Fig. .11, illustrating diagrammatically the structure of a Craspedote Medusa. 



A, a radial, B, an inter-radial section. 

 The ectoderm is indicated by a broken, line. The dashes are thicker where it consists of columnar 



cells (over n in B), or where it developes muscular fibres. The endoderm is indicated by a 



thick dark line. 



The shape is that of a bell, from the concavity of which depends a hollow 

 tubular process, the manubrium (M.\ At the free end of the manubrium is the 

 mouth (m.) ; at its base a cavity more or less dilated, the atrium (a.). From this 

 atrium radial canals (A: r.c.), varying in number, pass outwards in the substance of 

 the bell, and are united by a circumferential canal (A: c.c.\ which runs circularly 

 round its apparent edge. The true edge or rim of the bell is turned horizontally 

 inwards, and forms the velum (#.). The whole system of cavities is lined through- 

 out by a ciliated endoderm, represented by a thick dark line, the cells of which 

 vary in size and character in different regions. This endoderm is everywhere sepa- 

 rated from the ectoderm, represented as a broken line, by a supporting lamina. 



