252 C. L. Beecher — Origin and Significance of Spines. 



The apertural spines on some of the Graptolites are on the 

 most exposed portions of the hydrotheca, as in Monograptus 

 spinigerus™ Dioranograptus Nicholsowi™ Retiograptus ten- 



taculatas and Grapto/i/// m 7 uadrim ucronatus. In many com- 

 pound corals, the corallites are polygonal from crowding, and 

 the most exposed portions, the angles of the caliees, often bear 

 spines, as Favosites spinigerus, 30 Callopora exsul 30 etc. The 

 spines on the septa and costae of corals probably originate 1»\ 

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

 enced directly by external stimuli. 



The spines on the ventral sacs of Crinoids are usually termi- 

 nal, and in the most exposed situations; as in Scytalocrinus 

 validus™ Dorycrhms unicornis™ Aulocrinus Agassizi™ etc. 



The anterior and posterior pairs or rows of spines on the 

 loricse of some species of Rotatoria are in the most exposed 

 places ; as in Anurcea squamula, Noteus quadricornis, etc. 

 The spinules on the tubes of Spirorbis are usually developed 

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

 all sides; as Spirorbis spinuliferus. bl 



The spinules at the corners of the angular cell apertures of 

 many Bryozoa are in the most exposed situations, and probably 

 arise through external stimuli ; as in Trematopora echinata™ 

 T. spimdata™ etc. The large marginal spines of the Brachio- 

 pod Atrypa hystrix 31 probably owe their excessive develop- 

 ment to external stimuli, though the phylogeny of the species 

 shows that the spines first originated through the differentia- 

 tion 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. Exam- 

 ples of spines on post-umbonal slopes may be seen in Callista 

 siiblamellosa and young Saxicava arctica (figure 27). Such 

 spines represent periodic extensions of the mantle border, and 

 in some cases, the stimulus for this growth may come from 

 internal causes. The spines on TJnio spinosus and related 

 species are believed by Mr. Charles T. Simpson to assist in 

 anchoring the shell in the sand of swift running streams. In 

 Callista, the young Saxicava, and the Unios mentioned, the 

 spines occur on all individuals and at such an early period as to 

 preclude any special sexual function. 



In the Gastropoda, the periodic extension of the shell over 

 the posterior canal and the spiniform prominences formed on 

 the labrum are situated in exposed places, or where the amount 

 of stimulus is greatest ; as in Trophon magellanicus, Strombus 

 pugilis, Fusus colus, Clavatida mitra, Melo diadema, etc. 



The spines on the larvae of geometrid moths are usually on 

 top of the loop, and are explained by Packard 54 as follows: 



