HYDROZOA CRASPEDOTA. 749 



ectodermic origin, but sometimes, especially in the degenerate forms of sexual 

 zooid and the Siphonophora, formed in the endoderm. Marine with few 

 exceptions. 



There are three orders, Trachymediisae, the Hydroidea, and Siphono- 

 phora. 



The order Trachymedusae, the Trachylinae or second sub-legion of 

 Craspedota of Haeckel, contains Medusae which possess tentacles with a 

 solid axis, sometimes replaced in the adult by hollow tentacles, and ten- 

 taculocysts or auditory tentacles, with an axis of endodermal otolith cells. 

 Ocelli are rare. Development is direct by metamorphosis from free hydroid 

 larvae, and there is an Alternation of Generations only in one parasitic 

 species. ' Sporogony,' or development from a non-sexual spore, occurs in 

 a few instances. There are two sub-orders, the Narcomedusae and Tracho- 

 medusae. 



In the Narcomedusae, the exumbrella is flattish, rarely bell-like, and is 

 divided into a central and peripheral part by the insertion of the tentacles. 

 Its substance is firm and traversed by radial, branched and anastomosing 

 fibres. The peripheral portion is divided into lobes, one lobe between 

 every two tentacles. The lobes are connected together by the subumbrella 

 and by well-marked intervening streaks or 'peronia,' of ectoderm containing 

 cnidoblasts, continuous with a marginal band of cnidoblasts and ciliated 

 sense-cells, and reaching from the latter to the bases of the tentacles. A 

 nerve extends from the outer ring beneath each peronium. In the Pegan- 

 thidae, however, the subumbrella is also divided, the peronia are very 

 rudimentary, and the lobes are united by the velum. The subumbrella 

 corresponds solely to the peripheral region of the exumbrella, the central 

 portion being occupied by the stomach. The velum is broad ; it is either 

 horizontal or hanging vertically downwards. In locomotion the lobes of 

 the bell are turned inwards with the velum. 



The tentacles may be in number only two, or four, or eight, but in the 

 majority more numerous, but never more than thirty-two. In the Aeginidae 

 their number follows a geometrical progression, but in the other three 

 families it is irregular. Their ectoderm consists of cnidoblasts and sensory 

 cells, some ciliated, some provided with stiff sense-hairs. The axial endo- 

 derm cells are developed in a single row, but may be numerous at the base, 

 and are continued as the tentacle-root for some distance along the ex- 

 umbrellar aspect of the stomach. The tentaculocysts may be only four in 

 number, alternating with the four tentacles in Cunantha, generally more 

 numerous, often several hundreds in number. They are inserted on the 

 marginal band of the lobes (supra). The auditory ectoderm cells have long 

 auditory hairs : the axial endoderm cells are in a single row, as a rule two 

 to four, rarely more in number. Of these one, or two to four, contain one 

 or more calcareous otoliths, as a rule of regular crystalline figure, rarely, as 



