HYDROIDA 11 



a. Hydrothecze small, radially symmetrical, cannot accommodate the contracted hydranth. Fa- 

 mily Halcciidcc. 



b. Bilaterally symmetrical hydrothecse, with obliquely set diaphragm. 



X. Diaphragm simple. Hydrothecse small, approximately radial structure, generally without 

 teeth ; rudimentary or stalked and mobile, two-chambered sarcotheca. Family Plumulariidoe. 

 XX. Diaphragm generally composed of two obliquely set portions; h)drothecse markedly 

 bilateral, large, most frequently toothed. Sarcothecse all or some one-chambered, sessile 

 and immobile. Family Aglaopheniidcr. 

 II. Polyps with a basally situate, ventral blind sack with low endoderm cells. Hydrothecae gene- 

 rally bilateral. (Family series Serlulariina). 



a. Hydrothecae without opercula. Family SynfheciidcB. 



b. Hydrothecse with opercular apparatus. Family ScrtulariidcB. 



2. Polyps with club-shaped or trumpet-shaped oral parts. (Family series Proboscoida). 



A. Hydrothecse large, radially symmetrical. Family Campatiulariidw. 



B. Hydrothecse small, thick-walled, and bilaterally symmetrical. Family SilicitlariidcB. 



II. Thecaphore Hydroids of the Northern Atlantic. 



Family series Hebellina nov. 

 Family Lafoeidae. 



Hydrothecse deep bell- to tube-shaped, radially symmetrical after their ground plan, stalked or 

 sessile, at times partly fused with the mother tube; diaphragma rarely present. The hydrothecse 

 without opercula; exceptionally, the distal end of the one wall in the hydrotheca may fold in over the 

 aperture. The colonies are stolonial or sympodial. The polyps are radially symmetrical, with conically 

 pointed oral part; the endoderm is divided into an oral and a gastral part; the gastral endoderm is 

 homogeneous. 



The structure of the polyps gives this family a primitive rank among the thecaphores, and 

 among its lowest genera Hebclla must be counted as taking a typical position; this genus is, how- 

 ever, especially distributed in the warmer seas. The genus has also been recorded from northern 

 waters, although evidently erroneously; its only northern representative should be llebrlln pocillum 

 (Hincks) but as it lacks the diaphragm, it cannot be allowed to remain in this genus, and its identity 

 will be dealt with later on. 



In course of time, a whole series has been set up with genera of Lafoeidcr^ and on going 

 through the list compiled by Stechow (1913 p. 44) we find that considerable weight is attached to 

 the accumulation of the gonangia in aggregates (scapus, coppinia) or their appearing singly. I have 

 recently (1917) in studying the coppinia of Gra7nmaria abietina^ set forth what we know up to the 



