438 CHARACTERS OF INVERTEBRATE ANIMALS 



of ciliated tentacles in the form of a double horse-shoe, within 

 the concavity of which is the external opening of the intestine. 

 This protrusible part of the body is drawn back or introverted 

 by a retractor muscle at the approach of danger, reminding one of 

 the arrangement which obtains in Sipunculus (p. 433), a further 

 resemblance to which animal is found in the approximation of 

 the two openings of the gut, an obvious convenience in a tube- 

 inhabiting animal. Within the translucent body the U-shaped 

 food-tube can be clearly seen, consisting of gullet, stomach, and 

 intestine. There are two small excretory tubes opening near the 

 tentacular crown. The nervous system consists of a nerve -ring 

 closely surrounding the beginning of the gullet, and thickened 

 on the side next the intestine into a bilobed brain- or cerebral 

 ganglion. The stomach is connected with the body-wall by a 

 fibrous cord i^funicle) in which are developed buds known as 

 statoblasts. In winter the colony dies down, to be replaced in 

 the following spring by fresh colonies to which these buds give 

 rise. 



Another very beautiful fresh-water form, the transparency of 

 which makes it an attractive object under the microscope, is 

 Lophopus crystallinus (fig. 270), in which the skeleton is gela- 

 tinous, and the entire colony, instead of being fixed, is able to 

 creep slowly along the surface of water-plants. 



Class II.— LAMP-SHELLS (Brachiopoda) 



These are exclusively marine forms, including only about 

 120 recent species, though their area of distribution (which in- 

 cludes the British seas) is extremely wide. In some of the 

 geological periods, however, they were extremely abundant, but 

 have gradually declined in importance from very early times. 



The most striking characteristic is found in the presence of a 

 bivalve shell, which for a long time caused them to be confounded 

 with bivalve Molluscs (pp. 328-338). Taking a typical Lamp- 

 Shell [Waldheimia or Terebratula) (fig. 271) as an illustration, 

 we shall find many well-marked differences from the members 

 of the last-named group. To begin with, the two valves are not 

 right and left, but upper and lower, and they are of unequal 

 size, the upper being the smaller. In side view the shell resembles 



