254 



LOWER INVERTEBRATES. 



Fig. 262. — Dia^ani showing the development of the 

 gills of a lamellibranch. 



arises, bears, at the sides, a pair of leaf-like or tentacular folds, the labial palpi, the 

 function of which is to direct and conduct currents of water to the mouth, the cilia 

 with which they are covered aiding greatly in this respect. It has been suggested 

 that these palpi represent the velum of the larva, but no known facts of embryology 

 confirm this view. They are, in reality, the greatly expanded upper and lower lips. 



The alimentary canal always traverses the whole length of the body, terminating 

 in a vent at the posterior end. Usually its course is much contorted, the intestine, in 

 some forms, passing through the ventricle of the heart. The oesophagus is short, and 

 communicates with a more or less spherical stomach, into which a voluminous paired 

 liver pours its secretion. The intestine is very long and convoluted. No organs of mas- 

 tication are present, and, if we make one pos- 

 sible exception, nothing that can be compared 

 to the lingual ribbon of other molluscs. This 

 exception is the crystalline style. This is a 

 transparent elastic rod, of unknown functions, 

 wliich lies in a pouch arising from the stom- 

 acli. Whether it be a representative of the 

 odontophore is very uncertain. 



The heart always consists of a median 

 ventricle, which forces the blood to all parts 

 of the body, and two auricles, one on either 

 side, which receive the blood from the gills 

 and pour it into the ventricle. The gills 

 possess a very complicated structure, but one which can without much difficulty be 

 reduced to a simple type. Of these organs there are 

 usually two on either side. Embryology shows us tliat 

 each of these gills is primitively made up of a series of 

 little tubes running down from the body wall. These 

 tubes then turn and grow back until they reach the inner 

 surface of the mantle, as shown in the adjacent figures. 

 The filamentary condition persists in some acephals, but 

 in others the adjacent filaments become united so that a 

 broad lamellar gill (whence the name Lamellibranehiata) 

 is the result. In some forms this union is produced by 

 bunches of hooked cilia on the sides of the tube, while 

 in others the walls of the branchial filaments become 

 solidly grown together. The blood from all paits of the 

 body gathers in a large tube at the base of the gills; 

 thence it passes down through one half of the little tubes, 

 and up in the other, to another vessel, whence it is con- 

 veyed to the heart. During this passage it is brought in 

 contact with the water, discharging its carbonic acid, and taking a new supply of 

 oxygen. 



The way in which the water is brought into the cavity of the mantle, and in con- 

 tact with the gill, is very interesting. The gills, and for that matter the whole inner 

 surface of the mantle cavity, are covered by innumerable little hairs, or cilia, which, 

 by their constant motion (always in one direction) create currents in the water, draw- 

 ing it in through the incurrent siphon, passing it over the gills, around to the mouth. 



Fig. 263. — Section througli gill 

 filaments of Mytllus, showing 

 the hooked cilia which fasten 

 them together. 



