BRACHIOPODA. 



237 



(a] Tentacles in a crescent Fresh water, Crisfatella, Lophopits, etc. 



(b] Tentacles in a circle Marine, except Paludicella ; Flustra, the 

 common sea-mat ; Membranipora, encrusting sea- weed, etc. ; Cellepora, 

 very calcareous ; Akyonidium, 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 metamorphosis 

 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 

 "head kidneys" of Annelid trochospheres. They are said to terminate 

 in flame cells like those of Platyhelminths. 

 In all these three respects the Entoprocta 

 differ from the Ectoprocta, and from the 

 Molluscoidea generally ; but the signifi- 

 cance of this is uncertain. 



Class BRACHIOPODA. 



The Brachiopods or Lamp-shells are 

 quaint marine animals, once very numer- 

 ous, 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 FIG. 113. Interior of Brachio- 

 has apparently modified both the position p 0( j shell, showing cal- 

 and the relations of the organs. There is 

 no real resemblance between a Brachio- 

 pod 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 by a calcareous skeleton. These arise in development from 

 the specialisation of a horseshoe-shaped " lophophore," 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 ganglion. 

 Sensory structures in many cases perforate the valves. Above the gut 

 lies the heart, which is connected with blood vessels. Two (or more, 

 rarely four) nephridia open near the mouth, and serve also as genital 



careous support for the 

 'arms." After Davidson. 



