Zoomorphology (1992) 112:143-153 Zoomorphology 9 Springer-Verlag1992 The compass depressors of Paracentrotus lividus (Echinodermata, Echinoida): ultrastructnral and mechanical aspects of their variable tensility and contractility I.C. Wilkie 1, M.D. Candia Carnevali 2, and F. Bonasoro 2 1 Department of Biological Sciences, Glasgow Polytechnic, 70 Cowcaddens Road, Glasgow G4 0BA, Scotland, UK 2 Dipartimento di Biologia "Luigi Gorini", Universitfi degli Studi di Milano, Via Celoria 26, 1-20133 Milano, Italy Received February 12, 1992 Summary. The compass depressors are bands of soft tis- sue which connect the compass ossicles of the echinoid lantern to the inner edge of the test. They are essentially ligaments with on one side a thin layer of muscle cells. The ligamentous component consists mainly of a parallel array of collagen fibrils with interspersed 12 nm microfi- brils. The most notable cellular constituents are granule- containing cell bodies and their processes which resem- ble the juxtaligamental cells that have been found in all echinoderm mutable collagenous tissues and which may control the tensility of these tissues. The muscle cells occupy about 8% of the total cross-sectional area of the compass depressor and are located in a richly innervated pseudostratified myoepithelium. When sub- jected to constant low loads in creep tests the compass depressor stretches to a fixed length beyond which there is no further extension. The length at this creep limit coincides with the maximum length to which the com- pass depressor is stretched by natural movements of the intact lantern. Stress-strain tests show that treatment with 1 mM acetylcholine or 100 mM K + ions can in- crease reversibly the stiffness of the compass depressor to an extent that cannot be due to contraction of the myoepithelium, suggesting that the mechanical proper- ties of the ligament are under physiological control. Ten- sion-length data on the myoepithelium suggest that it generates a maximum active tension when the compass depressor is stretched to the creep limit. The implications of these results for the function of the compass depres- sors are discussed. A. Introduction All regular sea-urchins possess a complex masticatory apparatus known as Aristotle's lantern. Although much is known of the functional morphology of this structure (e.g. Mfirkel 1979; Mfirkel etal. 1990; Lanzavecchia Correspondence to: I.C. Wilkie et al. 1988; Candia Carnevali et al. 1991) and certain aspects of its mechanical behaviour have been explored (Candia Carnevali et al. 1988; Andrietti et al. 1990), lit- tle attention has been paid to those components of the lantern that form the compass system. The compass system consists of: (1) five compass os- sicles which are located on the aboral (upper) surface of the lantern, (2) five compass elevator muscles which interconnect transversely the compasses, (3) ten liga- ments linking the compasses to the underlying rotular ossicles, and (4) ten compass depressors - straps of soft tissue which extend from the distal lobes of the com- passes to the interambulacral processes of the perignath- ic girdle (inner edge of the test) at its junction with the flexible peristomial membrane (Fig. 1). The compasses 2 mm I I f o ce / ed re pr Fig. 1. Diagrammatic representation of the lantern of Paracentrotus lividus. The peripharyngeal coelomic membranes are omitted. Only the soft tissue components of the right side have been included; on the left side the insertion facets of the muscles are indicated by stippling, cd compass depressor; ce compass elevator; co com- pass; pm peristomial membrane; pr protractor muscle; re retractor muscle; ro rotula. Arrow indicates one of the pair of ligaments that link the compass to the rotula