ORIGIN AND SIGNIFICANCE OF SPINES 45 



Some of the Infusoria have terminal spiniform processes, 

 which, by analogy with other forms, have probably developed 

 according to I; as Ccrafiuia tripos^ 0. longicorne,'^ Cfusus.^ 



The apertural spines on some of the graptolites are on the 

 most exposed portions of the hydrotheca; as in Monograptus 

 spiiiiijerus,^^ Dicnuiogriiptiis Nicholsoni,^"^ Retiograptvs tenta- 

 culatits, and Graptolithux quadrimucnmatus. In many com- 

 pound corals the corallites are polygonal from crowding, and 

 the most exposed portions, the angles of the calices, often 

 bear spines; as Favositcs spinigfirux,^^ Callopora exsul,^" etc. 

 The spines on the septa and costa3 of corals probably originate 

 bv intrinsic forces (B), since they are internal growths not 

 influenced directly by external stimuli. 



The spines on the ventral sacs of Crinoidea are usually 

 terminal, and in the most exposed situations; as in Scy- 

 taloerinus valiJus,^^ Dorycrinus unicornis,^^ Aulocritius Agas- 

 sizi,^^ etc. 



The anterior and posterior pairs or rows of spines on the 

 loricffi of some species of Rotatoria are in the most exposed 

 places; as in A^iurcea squamula, Noteus quadricornis, etc. 

 The spinules on the tubes of Spirorhis are usually developed 

 after it rises above the object of support so as to be exposed 

 on all sides; as iSjnrorbis spinuliferus.^^ 



The spinules at the corners of the angular cell apertures 

 of many Bryozoa are in the most exposed situations, and 

 proljably arise through external stimuli; as in Trematopora 

 echinata,^ T. spiculata,^ etc. The large marginal spines of 

 the brachiopod Atrypa hystrix^^ probably owe their excessive 

 development to external stimuli, though the phylogeny of 

 the species shows that the spines first originated through the 

 differentiation of the radiate and concentric ornaments. 



In many pelecypods the siphonal region receives a great 

 amount of stimulus, and the post umbonal slope is the part 

 most exposed. Along this slope are found many of the 

 spines, and generally the greatest differentiation of ornament. 

 Examples of spines on post-umbonal slopes may be seen in 

 Callista sublamellosa and young Saxieava arctica (figure 27). 



