300 DK- J. E. DUERDEJN" OX THE 



The polyps are easily detached from the marine plants to which they are adherent ; 

 indeed, in transferring the leaves from the sea, or from one jar to another, the polyps 

 wUl often slide off. It is douhtful if an actnal adherence really takes place, any more 

 than in the case of such organisms as Nudihranchs and Turhellarians. The presence of 

 a basal ciliation, to be described below, would iudicate that the contact with foreign 

 bodies is not very close. The extremely weak parieto-basilar muscles and absence 

 of basilar muscles are no doubt the structural features correlated with the incapacity for 

 firm fixation. In the laboratory specimens often free themselves, and, suspended in the 

 water with the tentacles fully expanded, may move slowly around, gradually coming 

 to rest asrainst the sides of the vessel. Probably their movements are assisted by the 

 strong ciliation on the base and column-wall. I have never observed any decided 

 rhythmic movements in swimm.ing. When free the base may be upwards, with the 

 tentacles hanging gracefully downwards ; or it may be in its normal lower position, 

 the tentacles then directed upwards and partly overhanging. 



Carlgren (1900) also mentions that the BoIocerohJes IIcMurrichi, obtained by 

 Dr. Stuhlmann from Zanzibar, was likewise found to be able to detach itself and carry 

 on swimming movements much like a Medusa. Like Buiiodeojjsis, the genus .Bolo- 

 cerokles is considered to belong to the primitive Protantliese, as is also Gonactinia, 

 another form capable of swimming. Probably such loosely adherent Actinians should 

 be regarded as in some ways intermediate between the free Anemones with a rounded 

 aboral extremity and foi'ms with a flattened firmly adherent base. 



Anatomy axp Histology. 

 Base. 



The base is thin-walled, all the three constituent layers being very narrow (PI. 25. fig. 2). 

 In sections it is usually -03 mm. across. The ectoderm is remarkable for the short, 

 coarse cilia with which it is covered, and which are recognizable in basal sections even 

 when unpreserved in other parts of the polyp. In most preserved polyps only traces of 

 the external ciliation on other parts of the body-wall persist, while the ectodermal 

 stomodseal ciliation is usually very obvious, and less so that of the mesenterial filaments 

 and of the endoderm of the coelomic cavity. Probably the basal ciliation assists in the 

 movements of the polyps over the leaves of the marine plants on which they settle ; 

 or it may help the polyp when it assumes the floating habit. Its presence is in- 

 compatible with any firm adherence of the polyps by their base. 



The basal ectoderm is constituted mainly of supporting cells, narrow granular gland- 

 cells, and a few clear gland-cells. A small nematocyst, measuring -012 mm., occurs 

 very sparingly. A rudimentary musculature and nerve-layer probably occur, but their 

 presence could not be determined with absolute certainty. 



The mesogiffia is narrow, and contains a few minute connective-tissue ceUs. Elsewhere 

 throughout the polyp the mesogloea preserves the same character, that of a narrow, 

 homogeneous layer, containing but a few connective-tissue cells. 



The basal endoderm varies much in thickness in different places. Where very narrow. 



