LAMP-SHELLS 



287 



(a) Tentacles in a crescent — Fresh water, Cristatella, Lophopus, etc. 



{h) Tentacles in a circle — Marine, except Paludicella ; Flustra, the 

 common sea-mat ; Memhranipora, encrusting seaweed, etc. ; Cellepora, 

 very calcareous ; Alcyonidium, gelatinous. 



The Entoprocta include the colonial Pedicellina, with a few allied 

 genera, also the non-colonial Loxosoma, in which the buds separate 

 as soon as they are formed. All the forms are stalked and minute. 

 The anus is included within the tentacular circle. In the meta- 

 morphosis of Pedicellina there is an elongation of the dorsal region of 

 the body, and a consequent approximation of the mouth and anus on 

 the shortened ventral surface. There is no apparent body cavity in the 

 adult, and the mesoderm arises from two primitive mesoblasts. The 

 nephridia are anterior, minute, and do not serve as genital ducts, but 

 resemble the protonephridia of Annelid trochospheres. They are said 

 to terminate in flame-cells like those of Platyhelminthes. In all these 

 three respects the Entoprocta differ from the Ectoprocta, and from the 

 Molluscoidea generally. 



Class Brachiopoda 



The Brachiopods or Lamp-shells are quaint marine animals, once 

 very numerous, but now decadent. The body is enveloped dorsally and 

 ventrally by two folds of skin or mantle ; these secrete a shell, usually 

 of lime, but sometimes organic. The development of this shell has 

 apparently modified both the position 

 and the relations of the organs. There 

 is no real resemblance between a Brachi- 

 opod shell and that of a bivalve Mollusc, 

 except that both consist of two valves. 

 In Brachiopods these lie dorsally and 

 ventrally ; in Lamellibranchs they are 

 lateral ; moreover, in Brachiopods the 

 ventral valve is usually the larger. It 

 is hardly necessary to say that the 

 Brachiopod organism is not the least 

 like a Mollusc. 



A considerable part of the space 

 between the valves of the shell is filled 

 up by two long " arms," which are 



coiled in a spiral, and often supported Fig. 156. — Interior of 

 by a calcareous skeleton. These arise Brachiopod shell, showing 

 in development from the specialisation calcareous support for the 

 of a horseshoe-shaped " lophophore," "arms."— After Davidson, 

 such as is characteristic of the Polyzoa. 



The mouth is placed between the arms, and opens into the ciliated 

 food canal. This may end blindly, or may be furnished with an 

 anus placed near the mouth ; in Crania the anus is dorsal and 

 posterior. The muscular system is well developed, the shell being 

 both opened and closed by means of muscles. There is a nerve- 

 ring round the gullet, with a slight brain and an inferior gang- 



