HISTOLOGY OF THE ECTODERM. 



117 



If we examine one of the large thread-cells from the tentacle of Cori/nr pmilla prcvionsly 

 to its being exposed to the conditions which result in the emission of its filament, we shall find it to 

 consist of an external perfectly transparent oval capsule with rigid walls, and of certain remarkable 

 contents which play au important part in the special function of the thread-cell (PI. IV, fig. 4). The 



Fig. 51. 



Hydrothecae of Aglaoplienia pluma, with hydrantbs and neraatopliores. 



A. Hydrotlieca, with extended hydranth and with the sareode contents of the nematophores quiescent. 

 a, extended hydranth; c, serrated mar<^in of hydrotheca; d, segment of tlie ramulus carrying tlie liydrotheca; 

 e, mesial or azygous nematophore; f^ lateral uematophore; g, lateral aperture through which tlie mesial 

 nematophore communicates with the cavity of the hydrotheca. 



B. Hydrotheca, with retracted hydranth and the sareode contents of the nematophores emitting pseudo- 

 podial prolongations, o, retracted hydranth; c, margin of hydrotheca; d, segment of the ramulus carryings 

 the hydrotheca; e, mesial nematophore, with its protoplasm projected in an irregular pseudopodial mass; 

 y, through its lateral aperture into the cavity of the hydrotheca; f^ lateral nematophore, with the commence- 

 ment of a pseudopodium. 



C. Same parts with pseudopodial processes more advanced, a, retracted hydranth ; c, margin of hydro- 

 theca ; rf, segment of ramulus ; e, mesial nematophore from which a long clavute process, g, of protoplasm is 

 projected into the cavity of the hydrotheca; fy lateral nematophore from which a long pseudopodium is 

 projected into the surrounding water. 



D. Same parts showing different states of extension of the pseudopodia. a, retracted hydranth ; c, margin 

 of hydrotheca; d, segment of ramulus; c, lateral nematophore with a branching process,^, of its sareode 

 projected into the cavity of the hydrotheca ; /, lateral nematophore with the pseudopodium entirely withdrawn. 



In all the figures a cluster of thread-cells is seen imbedded in the distal end of the protoplasm within the sheath 

 of the nematophore. 



capsule is completely closed, and its longer axis is occupied by a membranous tube somewhat wider 

 near the centre than at either end. At one end of this tube, its walls are continuous with those 

 of the capsule, and it is this part of the capsule which usually lies most superficially when the 

 thread-cell is imbedded in the ectoderm ; it may, for the convenience of description, be 

 distinguished as the anterior end. The opposite end of the axile tube loses itself in a perfectly 

 transparent mass which occupies nearly the whole of the posterior half of the cavity of the 

 capsule, and in which I have in vain sought for anything like definite structure. 



The peculiar phenomena, however, which characterise the evolution of the thread-cell, 

 together with observations upon the structure of the thread-cell in other species, render it 

 almost certain that the apparently homogeneous mass in which the posterior extremity of the 



rence may be made to Leidy (" Marine Invertebrate Fauna of the Coasts of Rhode Island and New 

 Jersey," in ' Acad. Nat. Sci. Philadelphia,' vol. iii, 1855), Gosse (' British Sea Anemones,' 1860), 

 Clark (Agass., ' Cont. Nat. Hist. United States,' vol. iv, p. 209), and more especially to Mcibius 

 (" Ueber den Bau, den Mechanismus, und die Entwickelung der Nesselkapseln," from the ' Trans- 

 actions of the Natural History Society of Hamburg,' 18C6), who has given us a very full account of 

 these bodies, which he Las studied chiefly in Caryophyllia Smiihii. 



