508 Causes and Course of Organic Evolution 



ated, or more commonly there is a central or terminal patch 

 or circlet of cilia, or cilia and bristles, that form the so-called 

 trochal disc; while around the widened margin is a circlet 

 of cilia. The region thus demarcated may produce two pairs 

 of sensitive antennse, a dorsally and a subventrally or ven- 

 trally placed pair, or one pair may be absent or absorbed, or 

 the members of each pair may show all stages of fusion. They 

 are often jointed, they usually bear one or more bristles of a 

 tactile nature, and they may be capable of protrusion and 

 retraction. Between and usually near these, one to two patches 

 of surface cells early sink in from the surface to form cerebral 

 ganglia, while two, three, or four, rarely more, eye-spots are 

 formed above or on either side of the ganglia, and receive 

 separate or fused nerves from them. 



The mouth opens beside the marginal circlet of cilia, and 

 toward the ventral surface of the organism, which very often 

 swims or explores the muddy bottom of its territory by keeping 

 the oral side toward it, so that the opposite side becomes a 

 dorsal region. The mouth passes into a gullet or pharynx 

 that is protrusible at times, and in this are chitinous teeth 

 arranged in definite fashion, as will be emphasized afterwards. 

 The oesophagus often has paired salivary glands opening into 

 it, and then enlarges below into the stomach. The alimentary 

 canal next receives in some forms secretions from two large 

 glands that may be biUary in function, and then passing back- 

 ward bends slightly upward to open as an anus, or it may, in 

 tube-forming genera like Melicerta, bend into a dorsal loop 

 which ends in an antero-dorsal anus. 



The ventral area below and in front of the anus usually 

 becomes a jointed attachable foot, or may have a ciliate rim, 

 or be expanded into a disc, while paired "cement glands" open 

 at its extremity. These glands can secrete an adhesive cement, 

 or a glutinous exudation that is gradually poured out as an 

 investment which enables many species to resist drying, heat- 

 ing, or cooling for months or years. Primitive kidneys take 

 the form of two longitudinal tubes with or without branches, 

 and that open into a cloaca or bladder posteriorly, but in their 

 inner end openings are provided with "flame-cells" of excretory 

 nature. The dioecious reproductive organs consist of ovary, 

 yolk gland, and oviduct in the female, and of testis, seminal 

 vesicle, and penis in the male. A seemingly degraded and 

 derived condition in the adult is the semi-obliteration of divi- 

 sion walls between the cells throughout, but this probably 

 represents a loose separation of the cell protoplasms, derived 

 hereditarily from an infusorian ancestry. 



