POL YZOABRA CHIOPODA. 225 



ectoderm, and consists of a ring round the mouth, and of a cord down 

 the left side of the body. There is a closed vascular system with 

 nucleated red cells. The body cavity is well-developed. The sexes are 

 united. The larva, known as an Actinotrocha, is a much modified 

 trochosphere. 



Phoronis, the only genus, is a worm-like marine animal, always found 

 enclosed in a fixed leathery tube, and social in habit. 



Class POLYZOA. 



As usually defined the class includes two sub-classes, the Ectoprocta 

 and the Entoprocta, but it seems doubtful whether the Entoprocta 

 should not be raised to the dignity of a distinct class. 



The Ectoprocta include fresh water and marine forms in which the 

 anus is outside the basis of the tentacles. The nervous system is 

 represented by a ganglion placed between the mouth and anus. There 

 is no vascular system. In Cristatella, at least, there are two nephridia. 

 All are colonial and bud very freely ; the marine forms show con- 

 siderable division of labour among the members of the colony. 



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



[b] Tentacles in a circle Marine, except Paludicella ; Fhistra, the 

 common sea-mat ; Membranipora, encrusting seaweed, etc. ; Cellepora^ 

 very calcareous ; Alcyonidtum, gelatinous. 



The Entoprocta include the colonial Pedicel Una, with a few allied 

 genera, and Urnatella, also the non-colonial Loxosoma, in which the 

 buds separate as soon as they are formed. All 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. In all these three respects 

 the Entoprocta differ from the Ectoprocta, and from the Molluscoidea 

 generally, but the significance of this is uncertain, more especially as it 

 is possible that the differences may in part arise from defective 

 observation. 



Class BRACHIOPODA. 



The Brachiopods or Lampshells 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 appar- 

 ently modified both the position and the relations of the organs. There 

 is no real resemblance between a Brachiopod 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. 

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