LAMP-SHELLS. 



The Lamp-Shells, — Class Brachiopoda. 



427 



We must now leave the moss-animals, about whose relationship to the rest of 

 the animal kingdom we know so little, and pass on to the equally enigmatical class 

 of the arm-footed animals, or Brachiopoda. The chief structural feature in the 

 Brachiopods which led to their being classed as molluscs, was their bivalve shell. 

 They were therefore regarded as a subdivision of the bivalves. How easily a 

 Brachiopod might be mistaken for a mussel will be understood by the reader who 

 glances at the accompanying illustrations. But whereas the shells of the mussel 

 are at the animal's sides, and close in front, behind, and below, the hinge being on 

 the back, in the Brachiopods one shell is on the back and the other underneath. 

 There are, in addition, other profound differences in the anatomy of the soft-parts. 

 A further argument against there being any relationship between the Brachiopods 

 and the mussels is found in the fact that no intermediate forms exist which could 

 facilitate the deduction of the one class from the other. On the other hand, many 

 zoologists are inclined to consider the Brachiopoda to be modified worms, a view 

 confirmed by their anatomy. Although there is little to record of the activities 

 of these creatures, they are worthy of attention, not only on account of their 

 structure, but also from their extraordinary stability. With regard to this latter 

 point they are almost unique in the animal kingdom (if we omit the lowest 

 unicellular organisms), in having remained essentially unaltered from the earliest 

 geological epochs. They have neither progressed nor degenerated, but have lived 

 on practically at a standstill, so far as organisation is concerned. The period in 

 which they flourished most is now long past. Not only in the number of species, 

 but also in the number of individuals were they once so rich that thick layers of 

 rock have been built up by their accumulated remains. Brachiopods are divided 

 into two orders ; those having shells without hinges, and those with shells hinged 

 together. Taking the latter order (Testicardines) first, a few of its most important 

 families may be described. 



At the present time, the most widely spread group are the 

 Terebratulidce or perforated Brachiopods, to which Terebratulina, 

 figured on p. 428, belongs. In all species of this family, the dissimilarity of the 

 two shell-valves is strongly marked, one valve being larger than the other, more 

 concave, and perforated at the beak. The hole through the beak in this family 

 resembles in some cases the hole for the wick in an ancient lamp, and has thus led 

 to the name lamp-shells being applied to all Brachiopods. Through the perfora- 

 tion a short sinewy stalk emerges, by means of which the animal attaches itself to 

 submarine objects. The hinge at the beak consists of a pair of teeth situated on 

 the larger valve, and fitting into depressions in the smaller valve. Thus, although 

 these shells do not possess an elastic band or ligament, like that which binds 

 together the shells of a mussel, they are prevented from falling apart. The valves 

 are shut and opened by means of muscles. In consequence of the position of the 

 animal and of its organs, the larger more concave valve has been called the ventral 

 valve, and the smaller the dorsal valve or lid. The most remarkable feature in 

 these ancient forms is the looped calcareous framework attached to the under 

 surface of the lid near the hinge, and running forward towards the gape of the 



