202 



PHYLUM CCELENTERA 



I. Order Hydromedusae.— Simple or colonial forms in which the 

 sexually reproductive persons are either liberated as free-swimming 

 medusoids or are sessile gonophores. 



(a) Hydrophora.— Two types are included here. The first mcludes 

 the Tubularians, Hydractinia, and other forms in which the polyps are 

 not enclosed in the protective 

 perisarc which often surrounds 

 the colony (gymnoblastic), and 

 in which the free medusoid 

 forms, when present, have 



Fig. 107. — Diagram of a gymno- 

 blastic Hydroid. — After All- 

 man. 



a. Stem ; b, root ; c, gut cavity ; d, 

 endodcrni (dark) ; e, ectoderm ; /, 

 horny perisarc ; g, hydra - like 

 " person " (hydranth) ; g', the 

 same, contracted ; h, hypostome 

 bearing mouth ; k, sac-like repro- 

 (hictivebud (sporosac) ; /.medusoid 

 " person " ; m, a modified hydranth 

 (blastostyle) bearing sporosacs. 



I. 



i 



Fig. 108. — Graptolites. 

 I. Monograptus. II. Diplograptus. 



their genital organs placed in the wall of the manubrium (Antho- 

 medusa;), and are furnished with ocelli placed at the base of the 

 tentacles. Hydra and its allies may be included here. 



An unattached marine hydroid — Hypolytus peregrinus — has been 

 described, and as it bore gonophores it was obviously mature, which 

 is doubtful as regards two other unattached forms, Protohydra leuckartii 



