The invertebrates ability to adapt to the environment during motion represents an intriguing feature to inspire robotic systems. We analysed the sipunculid species Phascolosoma stephensoni (Sipunculidae, Annelida), and quantitatively studied the motion behaviour of this unsegmented worm. The hydrostatic skeleton and the muscle activity make the infaunal P. stephensoni able to extrude part of its body (the introvert) from its burrow to explore the environment by remaining hidden within the rocky substrate where it settled. The introvert protrusion is associated with changes in the body shape while keeping the overall volume constant. In this study, we employed a marker-less optical tracking strategy to quantitatively study introvert protrusion (i.e. kinematics, elongation percentage and forces exerted) in different navigation media. When P. stephensoni specimens were free in sea water (outside from the burrow), the worms reached lengths up to three times their initial ones after protrusion. Moreover, they were able to elongate their introvert inside a viscous medium such as agar-based hydrogel. In this case, the organisms were able to break the hydrogel material, exerting forces up to 3 N and then to navigate easily inside it, producing stresses of some tens of kPa. Our measurements can be used as guidelines and specifications to design and develop novel smart robotic systems.

无脊椎动物在运动过程中适应环境的能力代表了激发机器人系统的有趣特征。我们分析了 sipunculid 物种Phascolosoma stephensoni (Sipunculidae, Annelida)，并定量研究了这种未分段蠕虫的运动行为。静水骨骼和肌肉活动使动物群P。斯蒂芬索尼能够将其身体的一部分（内向者）从它的洞穴中挤出来探索环境，方法是隐藏在它定居的岩石基质中。内向突出与身体形状的变化相关，同时保持整体体积不变。在这项研究中，我们采用无标记光学跟踪策略来定量研究不同导航媒体中的内向突出（即运动学、伸长率和施加的力）。当P . 斯蒂芬索尼标本在海水中游离（在洞穴外），蠕虫伸出后的长度可达初始长度的三倍。此外，他们能够在粘性介质（例如基于琼脂的水凝胶）中拉长内向的人。在这种情况下，生物体能够破坏水凝胶材料，施加高达 3 N 的力，然后在其中轻松导航，产生几十 kPa 的应力。我们的测量结果可用作设计和开发新型智能机器人系统的指南和规范。