CHAP, iv.] THE CRUISES OF THE 'PORCUPINE? 167 



Operculince of tropical seas, and Biloculince and 

 Triloculince far exceeding in dimensions the littoral 

 forms of British shores; and with these were asso- 

 cated Cristellarice of no less remarkable size, pre- 

 senting every gradation from an almost rectilineal 

 to the nautiloid form, and having the animal body in 

 so perfect a state as to enable it to be completely 

 isolated by the solution of the shell in dilute acid." 

 Sponges were extremely abundant, but they were 

 restricted to only a small number of species ; all of 

 them with one form or another of the curious an- 

 choring habit. Among the Hexactinellidee Holtenia 

 was the most striking and the most abundant form. 

 Hyalonema was also common ; but we got few per- 

 fect specimens with the sponge and glass-rope in 

 connection. The conical sponge heads were very 

 numerous ; they seemed to have been torn off by 

 the edge of the dredge, the rope remaining in 

 the mud, and the ropes were frequently brought 

 up without the sponge. Almost all the ropes were 

 encrusted with the constant ' commensal ' of Hya- 

 lonema, Palythoa fatua. Very young examples of 

 Hyalonema, with the whisp from 5 mm. to 20 mm. 

 long, had usually no Palythoa on them ; but when 

 they had attained above the latter dimensions in 

 almost every case one could see the first polyp of 

 the Palylhoa making its appearance as a small 

 bud, and its pink-encrusting coenosarc spreading 

 round it. By far the most common sponge in the 

 chalk-mud is the pretty little hemispherical corti- 

 cate form Tisiphonia agariciformis. This species, 

 though differing from it greatly in appearance and 

 habit, seems to be closely allied to a strong, heavy 



