358 iMioF. E. A. Mixonix ox THK BRITISH [Dec. 13, 



ai-e subject to consideiMble variatioii.s, due to the particvilar en- 

 vironment and conditions under which the sponge grows in each 

 case. The variability of form is, however, combined always with 

 constant and characteristic features, and may be compared with 

 the different foi-ms which a creeping plant will assume under 

 different natural conditions. Too much has been made, in many 

 works, of this variability, and not enough of the constancy which 

 underlies it. No one Avho has a working acquaintance with 

 Ascons can ever mistake a Leucosolenia for a Clathrina, except 

 perhaps in the very youngest stages of growth ; there is no need 

 for me to repeat here what I have said in previous memoirs a>s to 

 the diagnostic importance of the form of the sponge foi' dis- 

 tinguishing the two genera. It is, moreover, by no means difficult,, 

 indeed it is usually very easy, to recognise at sight all the species 

 occurring in any pai-ticular localit3% Avhen one has once l^ecome 

 intimate with their characteristic features. Yet from looking at 

 Haeckel's plates of these sponges, the impression gained is that 

 Ascons have no characteristic generic or specific features except 

 in the sjiiculation. Haeckel's artistic pencil has misled him, and 

 others, upon this point, and his plates fail to portray the natural 

 appearance of the sponges. 



Speaking generally, there are three principal types of bod^'-f orm 

 occurring in the species of Leucosolenia. If the sponge be growing 

 on a bare rock, or on the stem of a large alga, it will creep over 

 it, sending out anastomosing basal stolons from which oscular 

 tubes arise at intervals. Such a specimen has been figured by me 

 elsewhei'e [16, fig. 5] ; it was found growing over a granite rock at 

 Roscoff, and is now in the British Museum. This type of colony 

 may be designated the sjyreading form ; it is not at all common, 

 since it may be supposed to be a rai-e occurrence for the sponge to 

 find a rock-surface unoccupied by other competitors. More usually 

 these sponges are found growing crowded up amongst algse and 

 various organisms, often in muddy situations, or creeping over 

 the seaweeds, and they then assume a form which may be teiined 

 hushy ; an example of this mode of growth has been figured by 

 me in the case of a specimen of L. lieherkuhnii [1. c. fig. 3] which 

 came fi'om the keel of a ship moored permanently in the Porto 

 Militare at Naples, and wdiicli was growing in a luxurious forest 

 of algge, hydroids, barnacles, worm-tubes, &c. The bushy form is 

 the commonest type of Leucosolenia-colonj . In a third modifica- 

 tion the sponge forms a creeping or arhorescent groAvth usually 

 closely applied to its support, but sometimes branching out under 

 favourable circumstances into tree-like growths [1. c. fig. 4]. 

 Although all these three modifications of form merge into one 

 another, it is convenient to classify them into the three principal 

 types noted above. 



A few words upon the characters of the spicules will not be out 

 of place here. The three kinds of spicules found in Leucosolenia 

 may be classified into: (1) monaxon spicules, simple needle-like 

 forms; (2) triradiate systems, with or without the addition of a 



