THYEOSTRACA 11 



tail-spine. Some authorities hold that this order should 

 be removed from the crustacean class to that of the Arach- 

 nida. The name of the third order, the Trilobita, refers to 

 the circumstance that they usually have the body divided 

 by two longitudinal dorsal grooves into three lobes. They 

 were extremely abundant in bygone ages, and the natu- 

 ralists of the Challenger were continually in hopes that 

 they might obtain a living specimen or two from hitherto 

 unexplored abysses of ocean. But extinction appears to 

 have done its work with great thoroughness upon this 

 order. 



The last of the sub-classes consists of the Cirripedia or 

 curl-footed animals. The alternative name Thyrostraca, 

 meaning ' valve-shells,' has the merit of agreeing in ter- 

 mination with the names of the other three sub-classes. 

 But it must be admitted that if it is objectionable to call 

 the whole group cirripedes when some have no cirri, it is 

 equally inappropriate to call them all ' valve-shells' when 

 some have no valves. It is a triumph of the present cen- 

 tury in minute investigation and comparative anatomy, 

 that has withdrawn the Cirripedes from the zoophytes, 

 worms, and molluscs, among which, at various times, the 

 older naturalists placed them, and that has given them 

 henceforth an undoubted position among the Crustacea. 



They may be divided into five orders, or the first two, 

 the Pedunciilata and the Operculata, may be grouped 

 together as divisions of an order hitherto designated 

 Thoracica, in which the part called the thorax is provided 

 with cirri. The AbdomiDalia have the cirri only on the 

 so-called abdomen. The Apoda are without cirri, being, 

 as their name implies, footless. Lastly, the Rhizocephala 

 are a parasitic set, which send rootlike filaments into the 

 bodies of their hosts. 



