26 THE VOYAGE OF H.M.S. CHALLENGEE. 



somewhat larger oval form, were surrounded by a perfectly continuous skin, and exhibited 

 no trace of a connecting stalk. The chamber layer in the former case was still simjjly a 

 closed sphere, while in the second a rupture had taken place towards the pointed pole of 

 the oval body, at the point apparently where the osculum would be developed. A 

 beautiful post-embryonal series, with certain modifications of form, was observed by 

 Wj'ville Thomson* in Pheronema [Holtenia) carjxnteri. 



Generally speaking, however, no important modification in the form seems to occur 

 during the development. In those Lyssacina in which the spicides are never soldered 

 together, the growth may apparently continue until the death of the animal ; so that 

 giant forms of 50 cm. in diameter and more may arise, as Poliopogon gigas, Mala- 

 cosaccus vastus, &c. In many Lyssacina, however, which, as they grow, typically 

 exhibit a soldering of the principal needles, there appears to be a definite limit of 

 growth (e.g. in Euplectella aspergillum). It is readily intelligible that when the 

 soldering of the needles has progressed from the median portion of the tube to 

 the basal tuft on the one hand, and to the firm terminal sieve-plate on the other, 

 any further extension of the sponge-body is really impossible. It is difierent with the 

 Dictyonina, in which the principal needles (Dictyonalia) are immediately after their 

 formation united with one another into a connected framework. Here, however, both on 

 the free margin of the cup- or tube-like body, and on the whole dermal and gastral 

 surface, there is a persistent continuance of growth through the laying down of fresh 

 portions of the framework and simultaneous displacement of the loose dermal and gastral 

 skeleton. It is only when the dermal skeleton becomes itself rigid, through the union of 

 its spicules in a reticulate framework, or by a continuous siliceous membrane, that 

 an absolute check is placed upon any further lateral growth. This seems also to occur in 

 individual cases, e.g., in Fieldingia, and with apparent regularity in several fossil forms. 



In many species the whole body dies and falls to pieces at once, while in others 

 dissolution is more gradual and begins at the base. Thus many specimens, especially 

 of Dictyonina, and also many Lyssacina, are dead in their basal portion, which consists 

 sim2>ly of a perfectly macerated skeleton, representing either the loose fibrous tuft, as in 

 PoJiopogon amadou, or a dead portion of the dictyonal skeleton. The latter case is 

 illustrated in PI. GIL fig. 1, in reference to Euryplegma aurictdaris, where the lower 

 boundary of the persistent, somewhat darker soft body is sharply marked ofi". On a large 

 beautifully developed specimen (40 cm. in height) of Aphrocallistes vastus, which Dr. 

 Gotsche brought from Japan, the whole basal portion for about a hand's length has been 

 killed. The siliceous elements rooted in the mud are not, however, always wholly dead. 

 The long tuft needles of the Hyalonematidge and the anchor needles of the Euplectellid* un- 

 doubtedl}^ retain their life, being probably nourished through the strand of their axial canals. 



In all skeletal elements which have been exposed, for a lengthened period after 



' Phil. Trans., vol. clix. p. 70. 



