INTRODUCTION. 
The Tetractinellida offer for our study one of the most interesting of all the groups 
of sponges : commencing with such comparatively simple forms as Placina and Tetilla 
they culminate in Geodia and the Stellettidae, which are the most highly organised 
representatives of the Parazoa ; at the same time they are connected in so nicely a 
graduated series as to form a highly natural group, within which the problem of tracing 
the evolution of structure from lower to higher stages may be attempted with much hope 
of success. 
The singular and striking characters of the higher forms have rendered them objects 
of interest from very early times, so that of recent Choristids faithful descriptions will 
be found in the works of Donati, published so far back as 1758, and of the fossil Lithistids 
good accounts occur still earlier, Guettard in 1751 describing in great detail the petrified 
pears or fossil figs of earlier writers, now known as Siphonia. These he not unnaturally 
assigned to the Corals, a mistake that Gray was near repeating when the first described 
recent Lithistid (Macandrewia) came before him for classification. 
From these early times to the present naturalists have constantly added to our know- 
ledge of the group, but no one up to the date of the commencement of this Eeport has 
published an account of their fundamental structure, if we except the important work 
of F. E. Schulze on the Placinidse and several descriptions of Astrophorous Sponges by 
myself ; while during its progress but one memoir on the anatomy of a single form has 
made its appearance. Nor, considering the general inaccessibility of most of the species, 
is this to be wondered at ; and the value of the fine collection brought home by the 
Challenger lies not so much in the addition of new and remarkable forms to the group, 
though these are not wanting, as in the presence of well-preserved examples of nearly 
every important genus ; so that for the first time it has become possible to publish a 
system of the Tetractinellida founded on a knowledge of their anatomy ; and only by such 
knowledge extending over a large series of species could a consistent system be founded 
with any chance of success. 
Of the defects in the proposed Classification no one can be more conscious than 
myself ; most especially do I regret the uncertainty which attends the phylogeny based 
