TARIETT OF ALECTOJ^A MILLABI. 535 



It is obvious that any individuals which tended to grow in two 

 directions only would stand a very much better chance o£ sur- 

 vival, and that the continued selection of such individuals would 

 result in highly specialized shell- boring sponges like Gliona 

 Fryeri (Hancock), which burrows through Placuna shells of 

 extreme thickness, establishing communications for each chamber 

 on both sides, without setting up any irritation of the mollusc 

 and consequent deformation of the shell. 



The Spicules. — The Skeleton-Spicules or Megasclera are diac- 

 tinal and pointed at the end {oxeas). They measure -J^ inch in 

 length, and in breadth -3-^ inch *, and the surface is covered 

 with sharp conical spines ; they are therefore somewhat smaller 

 than those of A. Millari, Carter, and are rather more strongly 

 spined. 



The Flesh- Spicules or Microsclera are diactinal, consisting of 

 a slightly bent shaft, blunt at the ends, bearing two whorls of 

 papillae, each situated at about one third of the length of the axis 

 from the end t. 



There are two forms of these microsclera : — (a) Larger ones in 

 which the average length of the axis is -J-y inch and the pa- 

 pillge are sessile. (6) Smaller forms only -j-^-q of an inch in lengtl], 

 with the whorls of rounded knobs carried out from the axis on 

 short stalks, and having consequently greater proportional 

 width. These two types are well defined and do not graduate 

 into one another. Both were present ia A. Millari, but 

 apparently only the smaller in the form first noticed as 

 " Gummina JVallichii.^' Fragments of a homogeneous membrane 

 which resist the action of acid are full of the small microscleres, 

 and probably are remains of a dermal layer. 



There is a third kind of spicule present in the form of slender 

 rapTiides. These were also present in A. Millari, and commonly 

 occur in groups of two or three lying at definite angles to one 

 another, connected by dried sareode in which tlie smaller micro- 

 scleres also occur in bands forming a sort of network. Possibly 

 these bodies are gemmae. 



The most important difierence then between this variety and 



* The dimensions are given in inches to compare with the original type. 



1" The flesh-spicules of the original specimen were Wuut-euded though repre- 

 sented as acute. Mr. Carter asks me to state that though his initials were 

 appended to the plate accompanying the description (Journal Eoyal Micr. Soc. 

 1879, pi. xvii.), this was a mistake, and he is only responsible for pi. xvii. a. 



